Drop return value from RtpRtcp::IncomingRtcpPacket.

And from its callee RTCPReceiver::IncomingPacket.

BUG=webrtc:5565

Review-Url: https://codereview.webrtc.org/3009403002
Cr-Original-Commit-Position: refs/heads/master@{#19823}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 479d3d75df72a46956f1336cb969085080064971