Remove UpdateSsrcs from EncoderStateFeedback.

Removes ability to modify set SSRCs from EncoderStateFeedback after
construction.

BUG=webrtc:1695
R=sprang@webrtc.org
TBR=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1241123002

Cr-Commit-Position: refs/heads/master@{#9603}