OpenACC: Remove 'acc_async_test' -> skip shortcut in 'libgomp/oacc-async.c:goacc_wait'

We're not taking such a shortcut anywhere else, and (with future changes) it
has potential to confuse things if synchronization in a libgomp plugin happens
to have side effects even if an async queue currently is empty.

	libgomp/
	* oacc-async.c (goacc_wait): Remove 'acc_async_test' -> skip
	shortcut.