For muxers, the internal AVCodecContext is basically unused
except in avformat_transfer_internal_stream_timing_info()
(which sets time_base and ticks_per_frame) and
av_stream_get_codec_timebase() (a getter for time_base).
This makes ticks_per_frame write-only, so don't set it.
Also remove an always-false check for the AVCodecContext's
codec_tag.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>