Delete VP8 feedback mode.

It depends on RTCP RPSI and SLI messages, which are being deleted.

TBR=stefan@webrtc.org  # TODO comments added to common_types.h
BUG=webrtc:7338

Review-Url: https://codereview.webrtc.org/2753783002
Cr-Original-Commit-Position: refs/heads/master@{#17314}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3257b16156ee3065edbc722c6e215b58a8914ad2
13 files changed