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