Remove VCM_*_PAYLOAD_TYPE constants.

These payload types aren't directly connected to any payload type, and
the payload type still has to be negotiated externally. As such these
constants are just a source of confusion.

BUG=
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1215603003

Cr-Commit-Position: refs/heads/master@{#9546}
4 files changed