### Please describe the problem.

even if url is associated with `web` remote, if some other remote can claim it, `git annex rmurl` would announce it not available for a "custom remote" (with `:` prefix) thus not `web`, thus keeping it "available" in subsequent calls to `git annex info`

<details>
<summary>complete example reproducer but needs datalad</summary> 

```shell
#!/bin/bash

export PS4='> '

set -eu
set -x

cd "$(mktemp -d ${TMPDIR:-/tmp}/dl-XXXXXXX)"
git clone http://github.com/ReproNim/containers
cd containers

git annex initremote datalad externaltype=datalad type=external encryption=none autoenable=true
git annex enableremote datalad

git annex info

git annex whereis images/bids/bids-rshrf--1.4.4.sing
git annex rmurl images/bids/bids-rshrf--1.4.4.sing 'https://storage.googleapis.com/singularityhub/singularityhub%2Fgithub.com%2FReproNim%2Fcontainers%2Feb098949b1061510767ef444c57a4f74e592f9fc%2F61b8e91176d796614444515a484f2a2d%2F61b8e91176d796614444515a484f2a2d.sif?X-Goog-Algorithm=GOOG4-RSA-SHA256&X-Goog-Credential=153500186594-compute%40developer.gserviceaccount.com%2F20210223%2Fauto%2Fstorage%2Fgoog4_request&X-Goog-Date=20210223T213231Z&X-Goog-Expires=3&X-Goog-SignedHeaders=host& '

git annex whereis images/bids/bids-rshrf--1.4.4.sing
git show -p git-annex
git show git-annex:fe1/c89/MD5E-s116695071--61b8e91176d796614444515a484f2a2d.4.sing.log.web

```
</details>


which ends with following output where you can see URL being listed associated with `web` (not datalad), `rmurl` "succeeding" but announcing it `0` on a custom remote (prefix `:`) and then subsequent `annex info` still shows that it is still available from `web`.  I would have expected that .log to be adjusted reflecting prior information (i.e. without `:`) and thus be removed from `web`:

[[!format sh """
> git annex whereis images/bids/bids-rshrf--1.4.4.sing
whereis images/bids/bids-rshrf--1.4.4.sing (3 copies) 
  	00000000-0000-0000-0000-000000000001 -- web
   	23456fab-05d4-441e-8952-b8c6d90ad785 -- yoh@smaug:~/proj/repronim/containers
   	71c620b5-997f-4849-bb30-c42dbb48a51e -- [datasets.datalad.org]

  web: https://storage.googleapis.com/singularityhub/singularityhub%2Fgithub.com%2FReproNim%2Fcontainers%2Feb098949b1061510767ef444c57a4f74e592f9fc%2F61b8e91176d796614444515a484f2a2d%2F61b8e91176d796614444515a484f2a2d.sif?X-Goog-Algorithm=GOOG4-RSA-SHA256&X-Goog-Credential=153500186594-compute%40developer.gserviceaccount.com%2F20210223%2Fauto%2Fstorage%2Fgoog4_request&X-Goog-Date=20210223T213231Z&X-Goog-Expires=3&X-Goog-SignedHeaders=host& 
ok
> git annex rmurl images/bids/bids-rshrf--1.4.4.sing 'https://storage.googleapis.com/singularityhub/singularityhub%2Fgithub.com%2FReproNim%2Fcontainers%2Feb098949b1061510767ef444c57a4f74e592f9fc%2F61b8e91176d796614444515a484f2a2d%2F61b8e91176d796614444515a484f2a2d.sif?X-Goog-Algorithm=GOOG4-RSA-SHA256&X-Goog-Credential=153500186594-compute%40developer.gserviceaccount.com%2F20210223%2Fauto%2Fstorage%2Fgoog4_request&X-Goog-Date=20210223T213231Z&X-Goog-Expires=3&X-Goog-SignedHeaders=host& '
rmurl images/bids/bids-rshrf--1.4.4.sing ok
(recording state in git...)
> git annex whereis images/bids/bids-rshrf--1.4.4.sing
whereis images/bids/bids-rshrf--1.4.4.sing (3 copies) 
  	00000000-0000-0000-0000-000000000001 -- web
   	23456fab-05d4-441e-8952-b8c6d90ad785 -- yoh@smaug:~/proj/repronim/containers
   	71c620b5-997f-4849-bb30-c42dbb48a51e -- [datasets.datalad.org]

  web: https://storage.googleapis.com/singularityhub/singularityhub%2Fgithub.com%2FReproNim%2Fcontainers%2Feb098949b1061510767ef444c57a4f74e592f9fc%2F61b8e91176d796614444515a484f2a2d%2F61b8e91176d796614444515a484f2a2d.sif?X-Goog-Algorithm=GOOG4-RSA-SHA256&X-Goog-Credential=153500186594-compute%40developer.gserviceaccount.com%2F20210223%2Fauto%2Fstorage%2Fgoog4_request&X-Goog-Date=20210223T213231Z&X-Goog-Expires=3&X-Goog-SignedHeaders=host& 
ok
> git show -p git-annex
commit 0738371567c5f925070e7da4c01ffa44c97bf830 (git-annex)
Author: Yaroslav Halchenko <debian@onerussian.com>
Date:   Mon Mar 15 18:46:06 2021 -0400

    update

diff --git a/fe1/c89/MD5E-s116695071--61b8e91176d796614444515a484f2a2d.4.sing.log.web b/fe1/c89/MD5E-s116695071--61b8e91176d796614444515a484f2a2d.4.sing.log.web
index b3f20a7..5d25928 100644
--- a/fe1/c89/MD5E-s116695071--61b8e91176d796614444515a484f2a2d.4.sing.log.web
+++ b/fe1/c89/MD5E-s116695071--61b8e91176d796614444515a484f2a2d.4.sing.log.web
@@ -1 +1,2 @@
+1615848366.629329639s 0 :https://storage.googleapis.com/singularityhub/singularityhub%2Fgithub.com%2FReproNim%2Fcontainers%2Feb098949b1061510767ef444c57a4f74e592f9fc%2F61b8e91176d796614444515a484f2a2d%2F61b8e91176d796614444515a484f2a2d.sif?X-Goog-Algorithm=GOOG4-RSA-SHA256&X-Goog-Credential=153500186594-compute%40developer.gserviceaccount.com%2F20210223%2Fauto%2Fstorage%2Fgoog4_request&X-Goog-Date=20210223T213231Z&X-Goog-Expires=3&X-Goog-SignedHeaders=host& 
 1614115961.926618255s 1 https://storage.googleapis.com/singularityhub/singularityhub%2Fgithub.com%2FReproNim%2Fcontainers%2Feb098949b1061510767ef444c57a4f74e592f9fc%2F61b8e91176d796614444515a484f2a2d%2F61b8e91176d796614444515a484f2a2d.sif?X-Goog-Algorithm=GOOG4-RSA-SHA256&X-Goog-Credential=153500186594-compute%40developer.gserviceaccount.com%2F20210223%2Fauto%2Fstorage%2Fgoog4_request&X-Goog-Date=20210223T213231Z&X-Goog-Expires=3&X-Goog-SignedHeaders=host& 
> git show git-annex:fe1/c89/MD5E-s116695071--61b8e91176d796614444515a484f2a2d.4.sing.log.web
1615848366.629329639s 0 :https://storage.googleapis.com/singularityhub/singularityhub%2Fgithub.com%2FReproNim%2Fcontainers%2Feb098949b1061510767ef444c57a4f74e592f9fc%2F61b8e91176d796614444515a484f2a2d%2F61b8e91176d796614444515a484f2a2d.sif?X-Goog-Algorithm=GOOG4-RSA-SHA256&X-Goog-Credential=153500186594-compute%40developer.gserviceaccount.com%2F20210223%2Fauto%2Fstorage%2Fgoog4_request&X-Goog-Date=20210223T213231Z&X-Goog-Expires=3&X-Goog-SignedHeaders=host& 
1614115961.926618255s 1 https://storage.googleapis.com/singularityhub/singularityhub%2Fgithub.com%2FReproNim%2Fcontainers%2Feb098949b1061510767ef444c57a4f74e592f9fc%2F61b8e91176d796614444515a484f2a2d%2F61b8e91176d796614444515a484f2a2d.sif?X-Goog-Algorithm=GOOG4-RSA-SHA256&X-Goog-Credential=153500186594-compute%40developer.gserviceaccount.com%2F20210223%2Fauto%2Fstorage%2Fgoog4_request&X-Goog-Date=20210223T213231Z&X-Goog-Expires=3&X-Goog-SignedHeaders=host& 
"""]]

git annex `8.20210310+git49-g4d6f74477-1~ndall+1`


[[!meta author=yoh]]
[[!tag projects/repronim]]

> [[fixed|done]] --[[Joey]]
