Reason for revert:
This has to be reverted to enable reverting cl https://codereview.webrtc.org/3006063002/, which seems to have broken ulpfec.
Original issue's description:
> Delete Rtx-related methods from RTPPayloadRegistry.
>
> Delete methods IsRtx, IsEncapsulated and RestoreOriginalPacket.
>
> BUG=webrtc:7135
>
> Review-Url: https://codereview.webrtc.org/3006993002
> Cr-Commit-Position: refs/heads/master@{#19739}
> Committed: 5b4b522641TBR=stefan@webrtc.org,danilchap@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7135
Review-Url: https://codereview.webrtc.org/3011093002
Cr-Commit-Position: refs/heads/master@{#19742}