clean up url removal presence update

* rmurl: Fix a case where removing the last url left git-annex thinking
  content was still present in the web special remote.
* SETURLPRESENT, SETURIPRESENT, SETURLMISSING, and SETURIMISSING
  used to update the presence information of the external special remote
  that called them; this was not documented behavior and is no longer done.

Done by making setUrlPresent and setUrlMissing only update presence info
for the web, and only when the url is a web url. See the comment for
reasoning about why that's the right thing to do.

In AddUrl, had to make it update location tracking, to handle the
non-web-url case.

This commit was sponsored by Ewen McNeill on Patreon.

Command/Migrate.hs

@@ -17,7 +17,6 @@ import qualified Command.Fsck
 import qualified Annex
 import Logs.MetaData
 import Logs.Web
-import qualified Remote
 
 cmd :: Command
 cmd = notDirect $ withGlobalOptions [annexedMatchingOptions] $
@@ -78,9 +77,8 @@ perform file oldkey oldbackend newbackend = go =<< genkey (fastMigrate oldbacken
 			-- If the old key had some associated urls, record them for
 			-- the new key as well.
 			urls <- getUrls oldkey
-			forM_ urls $ \url -> do
-				r <- Remote.claimingUrl url
-				setUrlPresent (Remote.uuid r) newkey url
+			forM_ urls $ \url ->
+				setUrlPresent newkey url
 			next $ Command.ReKey.cleanup file oldkey newkey
 		, error "failed"
 		)