Remove rtcp_interval from cast_config

As the TODO in cast_config.h bu miu@, remove rtcp_interval, since it is
always be kDefaultRtcpIntervalMs.

BUG=
R=miu@chromium.org

Review URL: https://codereview.chromium.org/1012573002

Cr-Commit-Position: refs/heads/master@{#321523}
8 files changed