Revert "avcodec/mediacodecdec: wait on first frame after input buffers are full"

@xyz reported a regression on his Sony Xperia Z3 Tablet Compact where
playback would intermittently fail to start, essentially deadlocking in
the decoder. Bisecting narrowed down the issue to this commit, which was
meant as an optimization but is not necessary.

This reverts commit a75bb549.

Signed-off-by: Aman Gupta <aman@tmm1.net>
(cherry picked from commit 37c2cb6a)