Remove CaptureManager and related calls in ChannelManager.
Removed unused screencast APIs.

BUG=webrtc:5426

Review URL: https://codereview.webrtc.org/1757843003

Cr-Original-Commit-Position: refs/heads/master@{#11896}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: c11b18483749b7f37559221b8401d24b9f5229f5