Reland of Delete Rtx-related methods from RTPPayloadRegistry. (patchset #1 id:1 of https://codereview.webrtc.org/3011093002/ )

Reason for revert:
The cl this change depended on has now been successfully relanded.

Original issue's description:
> Revert of Delete Rtx-related methods from RTPPayloadRegistry. (patchset #3 id:40001 of https://codereview.webrtc.org/3006993002/ )
>
> 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: https://chromium.googlesource.com/external/webrtc/+/5b4b52264132eefba10bc6880871715f9692da90
>
> TBR=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}
> Committed: https://chromium.googlesource.com/external/webrtc/+/a64685325c2f8f51873b67ae8a91f94ffb19d70b

TBR=stefan@webrtc.org,danilchap@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:7135

Review-Url: https://codereview.webrtc.org/3012253002
Cr-Original-Commit-Position: refs/heads/master@{#19821}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f54573bd3b9774db6782af8de0f22242598cb974
7 files changed