Remove WebRTC-videocontenttypeextension field trial completely

BUG=webrtc:7420

Review-Url: https://codereview.webrtc.org/3003673002
Cr-Original-Commit-Position: refs/heads/master@{#19459}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 41cadbcb0a3535353a4b431b67c96798f8eed06d
3 files changed