Intent to unship: AudioStreamTrack and VideoStreamTrack

[Andreas Pehrson]

I would like to unship the MediaStreamTrack subclasses AudioStreamTrack and
VideoStreamTrack.

Firefox is the only major browser implementing these classes and they don't
add anything other than the type. They were removed from the spec in 2014,
https://github.com/w3c/mediacapture-main/commit/d94246ed30e359722d4c70970eba6fbf22069e78

The alternative way of finding out whether a MediaStreamTrack is for audio
or video is to use the kind attribute. This was implemented in Firefox
together with MediaStreamTrack, AudioStreamTrack and VideoStreamTrack and
has been in the spec since the very beginning.

With these classes not being implemented in other major browsers I foresee
very few problems in the wild. I searched through github and identified
one(!) project that relies on them that I have filed an issue on.

This work is tracked in bug 1377146.
https://bugzilla.mozilla.org/show_bug.cgi?id=1377146