demux_mkv, demux_lavf: don't select initial audio track

demux_mkv, demux_lavf: don't select initial audio track

Remove code that tries to select audio track during demuxer
initialization from demux_mkv and demux_lavf. Just leave audio
disabled at that point; the higher-level select_audio() function will
call the demuxer to switch track later anyway.

Removing this unneeded code also fixes use of these demuxers as the
main demuxer with -audiofile. Before the automatic track selection
would have enabled an audio track (if the file had any); as the main
demuxer was not used for audio the unused packets from this enabled
track would accumulate until they reached queue size limits.