mov: do not set avg_frame_rate in the demuxer

The track duration is often not reliable or is not the duration
represented by the number of frames. In those cases, avg_frame_rate
was reported incorrectly. Removing this code falls back to the
default calculation in avformat_find_stream_info().

This is a partial revert of commit c3aeaa540.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>