Remove restart method from VideoTrackSourceInterface.

This method is no longer needed.

Review-Url: https://codereview.webrtc.org/2306013002
Cr-Original-Commit-Position: refs/heads/master@{#14060}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a973f953863608dab7ac82b35df08cac499a2b0f
7 files changed