Remove non-standard mediacapture-insertable-streams/legacy

[gsnedders]

We shouldn't have tests lying around that don't match the spec; if vendors that have outdated implementations wish to test them, they can move them into their own repo.

We should probably add some historical test to ensure we have test coverage that the old spec is not implemented (though I realise this partially overlaps with some of the things in mediacapture-insertable-streams/tentative which test for support under the Window global).

cc @youennf (c.f. #44262), @alvestrand (who seems to be the person to touch most of the legacy tests most recently)