remove xmpp support

I've long considered the XMPP support in git-annex a wart.
It's nice to remove it.

(This also removes the NetMessager, which was only used for XMPP, and the
daemonstatus's desynced list (likewise).)

Existing XMPP remotes should be ignored by git-annex.

This commit was sponsored by Brock Spratlen on Patreon.

Assistant/WebApp/RepoList.hs

@@ -260,7 +260,7 @@ getSyncNowRepositoryR uuid = do
 	if u == uuid
 		then do
 			thread <- liftAssistant $ asIO $
-				reconnectRemotes True
+				reconnectRemotes
 					=<< (syncRemotes <$> getDaemonStatus)
 			void $ liftIO $ forkIO thread
 		else maybe noop (liftAssistant . syncRemote)