Remove ViEChannel simulcast lock.

Since the number of streams is now known on construction we can
initialize all RTP modules on construction. They are internally locked
so we don't nede a simulcast lock anymore.

BUG=1695
R=mflodman@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/52639004 .

Cr-Commit-Position: refs/heads/master@{#9577}
12 files changed