Remove unneeded GetMucMessageCount from the XmppClient.

The affected function was imprecise, because it called a playerlist update whenever a "chat" level message was received instead
of only updating the playerlist if it's actually influencing the displayed playerlist.
When there is a chat message, lobby subject change or user-role change, there is no need to update the list.

Differential Revision: https://code.wildfiregames.com/D671
Refs #3386, rP16997
Reviewed By: fpre / ffffffff