[media] Remove kMojoVideoDecoder and kRTCVideoDecoderAdapter

These features are enabled by default, and the old implementations are
ready to be removed.

Also removes RTCVideoDecoder because there is now no way to use it.

Bug: 522298
Change-Id: Iabbfd65dc6160674333871306ab5d3111d2977e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1577948
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653885}
15 files changed