codec/rfx: removed unnecessary WaitForSingleObject

The WaitForSingleObject call on TilePool's event is called with a zero time-out
interval and the event is a manual reset event ... thus no locking or waiting
is involved anyways and Queue_Dequeue may very well return NULL independently
of calling WaitForSingleObject which is already correctly handled.
Merge pull request #1379 from nfedera/fix-2013-07-30-01

codec/rfx: removed unnecessary WaitForSingleObject