videotoolbox: remove obsolete workaround

Remove the picture flushing workaround since the described behaviour is
obsolete after 34a548cc, which flushes
the pictures from the video output before flushing the decoder.

It also ensures that we don't call decoder_UpdateVideoFormat which will
update the video output without video context, leading to potential
spurrious failures with OpenGL interop expecting one.

Co-authored-by: Alexandre Janniaux <ajanni@videolabs.io>