Replace AbstractClient::doPerformInteractiveMoveResize() with a simpler solution

AbstractClient::doPerformInteractiveMoveResize() is only used by the
X11Client to reset a boolean flag when the client doesn't support sync
counters.

X11Client can call performInteractiveMoveResize() only in two cases: the
sync request timer expires or the client increments the sync counter.

This change removes the AbstractClient::doPerformInteractiveMoveResize()
function and adds a function to handle the case where the sync timer
expires explicitly. This removes a virtual function in the AbstractClient
and makes code more readable.