Remove now-obsolete code setting packet durations pre-muxing for CFR
encoded video.

Changes output in the following FATE tests:
* numerous adpcm tests
* ffmpeg-filter_complex_audio
* lavf-asf
* lavf-mkv
* lavf-mkv_attachment
* matroska-encoding-delay
  All of these change due to the fact that the output duration is now
  the actual input data duration and does not include padding added by
  the encoder.

* apng-osample: less wrong packet durations are now passed to the muxer.
  They are not entirely correct, because the first frame duration should
  be 3 rather than 2. This is caused by the vsync code and should be
  addressed later, but this change is a step in the right direction.
* tscc2-mov: last output frame has a duration of 11 rather than 1 - this
  corresponds to the duration actually returned by the demuxer.
* film-cvid: video frame durations are now 2 rather than 1 - this
  corresponds to durations actually returned by the demuxer and matches
  the timestamps.
* mpeg2-ticket6677: durations of some video frames are now 2 rather than
  1 - this matches the timestamps.