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/TransferScanner.hs

@@ -76,7 +76,7 @@ transferScannerThread urlrenderer = namedThread "TransferScanner" $ do
 	 -   to determine if the remote has been emptied.
 	 -}
 	startupScan = do
-		reconnectRemotes True =<< syncGitRemotes <$> getDaemonStatus
+		reconnectRemotes =<< syncGitRemotes <$> getDaemonStatus
 		addScanRemotes True =<< syncDataRemotes <$> getDaemonStatus
 
 {- This is a cheap scan for failed transfers involving a remote. -}