Delete unused support for vp8 partitions.

This also makes it possible to drop the RTPFragmentationHeader from
the class VCMEncodedFrame.

BUG=None

Review-Url: https://codereview.webrtc.org/2380933003
Cr-Original-Commit-Position: refs/heads/master@{#14455}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6f112cc136bb8da01434747b6b0d0d56f69f271d
8 files changed