Remove deprecated RTPPayloadStrategy
Remove deprecated set_use_rtx_payload_mapping_on_restore()
Remove unused headers

BUG=None

Review-Url: https://codereview.webrtc.org/2808743002
Cr-Original-Commit-Position: refs/heads/master@{#17623}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: ca31f175e1e14d14f5036e4a9d4ef13796c9b799