muc: fix roomDestroyed() callback
The previous site where the callback was invoked was only reached if
there was also a user status on the unvailable presence. But those are
not part of unavilable presences upon room destruction.
muc: only call userHasLeft() at one site
There is no need to duplicate that code. Also ensure that
userHasLeft() is invoked *before* the listeners are invoked, so that
e.g. isJoined() returns false in the listeners.
