Remove default video encoders for new video API.

Reduces stream creation time significantly. As a side effect also
removes default encoders for receive-only channels.

BUG=1788,1667
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/37049004

Cr-Original-Commit-Position: refs/heads/master@{#8356}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 40367f984b2922fbfcf58d7e485ac0ef59149768
12 files changed