ext/ffmpeg/gstffmpegdec.c: Remove some more overly clever code that does nothing but mess up timestamps.

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_video_frame),
(gst_ffmpegdec_chain):
Remove some more overly clever code that does nothing but mess up
timestamps.