Remove unimplemented VideoChannel code.

Also removing a lot of dead testcases that were copied over and made
sense in the old implementation, now they just take space.

BUG=
R=pthatcher@google.com, pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/1658533003 .

Cr-Original-Commit-Position: refs/heads/master@{#11450}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a6c39d99029e9787727c0b0c34fc8d13c8470c81
8 files changed