Remove rtcp_interval from cast_config

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

Committed: https://crrev.com/870abc95c02e4ac0ae3f14ed4d4178be4ec2f98e
Cr-Commit-Position: refs/heads/master@{#321523}

[zhuoyu.qian, 2015-03-16 03:31:18]

Hi miu,
PTAL, Thank you!

[miu, 2015-03-20 04:48:48]

lgtm

Thanks for the clean-up work here!  :)

[zhuoyu.qian, 2015-03-20 04:55:02]

The CQ bit was checked by zhuoyu.qian@samsung.com

[commit-bot: I haz the power, 2015-03-20 04:55:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1012573002/1

[commit-bot: I haz the power, 2015-03-20 06:01:56]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-20 06:03:02]

Patchset 1 (id:??) landed as
https://crrev.com/870abc95c02e4ac0ae3f14ed4d4178be4ec2f98e
Cr-Commit-Position: refs/heads/master@{#321523}