Exclude subtitle track during sampling

[veikk0]

Audio tracks are already excluded during sampling with -an, and subtitles should probably be too (-sn).

Makes no practical difference since subtitle tracks are tiny and AFAIK FFmpeg only copies only a single track by default. And only text-based subtitles seem to be copied, at least no image-based DVD subtitles were copied in my test. So I'm only asking because it bothers me.