Remove MediaStreamTrack worker exposure feature flag

Worker support for MediaStreamTrack (and MSTGenerator) isn't being
actively worked on, and the flags have been disabled everywhere for
quite some months.
Also remove the MediaStreamTrackInWindow stable launched flag, which
was only required to allow Worker exposure to be flag-guarded.

Bug: 1302819
Change-Id: I9127785aee92bfe29d3048cc344d7958954ea276
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4084786
Auto-Submit: Tony Herre <toprice@chromium.org>
Commit-Queue: Yoav Weiss <yoavweiss@chromium.org>
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Reviewed-by: Tove Petersson <tovep@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1080935}