do not include URL in index.xml because it will get out of date

As far as I can tell, the 'url' metadata in index.xml is not used at all by
the client.  In order to keep it up-to-date in the local repo, it would
have to regenerate index.xml and index.jar each time the IP address
changed.  That would mean a decent amount of work happening in the
background, all the update an unused field in index.xml.

src/org/fdroid/fdroid/localrepo/LocalRepoManager.java

@@ -298,7 +298,6 @@ public class LocalRepoManager {
         repo.setAttribute("pubkey", Hasher.hex(LocalRepoKeyStore.get(context).getCertificate()));
         long timestamp = System.currentTimeMillis() / 1000L;
         repo.setAttribute("timestamp", String.valueOf(timestamp));
-        repo.setAttribute("url", uriString);
         rootElement.appendChild(repo);
 
         Element repoDesc = doc.createElement("description");