Remove RTCP tests from channel_unittest.

RTCP is no longer handled by channels as of
https://webrtc-review.googlesource.com/c/src/+/152668.  The tests for
RTCP in channel_unittest.cc are flaky and now only cover the logic of
passing RTCP through a transport to a fake on the other side.

Bug: webrtc:10983
Change-Id: Ib85b79adf79ee1524460b906b93b3a0e085ca8c4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/156324
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Bjorn Mellem <mellem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29438}