Remove VCMProtectionCallback::SetRetransmissionMode()

The method has no callers. Its only implementation, in RtpVideoSender,
forwarded the mode to RTPSenderVideo::SetRetransmissionSetting() under
mutex_ to serialize it with packetization of encoded frames, so removing
it also removes one of the remaining reasons for the mutex.

VCMProtectionCallback is public API, but there are no known callers or
implementations outside of WebRTC, and the header asks other users not
to use the API yet. Downstream implementations of VCMProtectionCallback
need to drop their SetRetransmissionMode() override.

Bug: webrtc:42223727
Change-Id: Ibef15509dd7d6317155a781a91c07d8deabeb117
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/506201
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48793}