Removing AudioMixerStatusReceiver and ParticipantStatistics.

BUG=webrtc:497
R=ajm@chromium.org, andrew@webrtc.org, henrikg@webrtc.org

Review URL: https://codereview.webrtc.org/1216133004 .

Cr-Commit-Position: refs/heads/master@{#9647}
9 files changed