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.

Remote/Web.hs

@@ -100,7 +100,7 @@ uploadKey _ _ _ = do
 
 dropKey :: Key -> Annex Bool
 dropKey k = do
-	mapM_ (setUrlMissing webUUID k) =<< getWebUrls k
+	mapM_ (setUrlMissing k) =<< getWebUrls k
 	return True
 
 checkKey :: Key -> Annex Bool