Delete remnants of RTX support in voice_engine.

Receive logic in voe::Channel attempted to handle RTX if
RTPPayloadRegistry::IsRtx returns true, but the audio code never calls
the config methods (SetRtxSsrc or SetRtxPayloadType) required for that
to ever happen.

BUG=webrtc:7135

Review-Url: https://codereview.webrtc.org/3006913002
Cr-Original-Commit-Position: refs/heads/master@{#19633}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: da194e79c4a86869e8afccf9827c716f0b073fb1
3 files changed