Delete Rtx-related methods from RTPPayloadRegistry.

Delete methods IsRtx, IsEncapsulated and RestoreOriginalPacket.

BUG=webrtc:7135

Review-Url: https://codereview.webrtc.org/3006993002
Cr-Original-Commit-Position: refs/heads/master@{#19739}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 5b4b52264132eefba10bc6880871715f9692da90
7 files changed