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/Threads/MountWatcher.hs

@@ -146,7 +146,7 @@ handleMount urlrenderer dir = do
 	debug ["detected mount of", dir]
 	rs <- filter (Git.repoIsLocal . Remote.repo) <$> remotesUnder dir
 	mapM_ (fsckNudge urlrenderer . Just) rs
-	reconnectRemotes True rs
+	reconnectRemotes rs
 
 {- Finds remotes located underneath the mount point.
  -