Remove MockRemoteBitrateObserver (unused)

BUG=None

Review-Url: https://codereview.webrtc.org/2731523002
Cr-Original-Commit-Position: refs/heads/master@{#17060}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: fb1fa44d704e9f1dbec7408773c1b0d1a2398e02
5 files changed