Remove xmpp_messaging plug-in (Replaced by new plug-in)

I've just released Messaging 6.x-2.0 which includes a new XMPP plug-in so I think we better drop the one here.

This new module besides a lot of polishing and taking advantage of XMPPFramework 6.x-2.x new connection handling has some new options for sending messages through a different method when the user is offline.
(We can see about the queueing option, which I've drop for now, when we have better support for it in the framework, but right now I think without proper handling we better don't queue messages...)