Remove SetOptions and other unused methods from channel interfaces

Delete SetOptions from VoiceMediaSendChannelInterface,
VoiceMediaReceiveChannelInterface, and VideoMediaSendChannelInterface,
as they lack any external or polymorphic callers.

* Remove unused implementations and associated internal logic from
  WebRtcVideoSendChannel, WebRtcVideoSendStream, and
  WebRtcVoiceReceiveChannel.
* Retain WebRtcVoiceSendChannel::SetOptions as a private non-virtual
  helper function.
* Mark internal tracking fields (default_send_options_ and options_)
  as const and drop their RTC_GUARDED_BY thread annotations, as they
  no longer undergo mutations or require explicit synchronization.
* Remove an unused VideoOptions parameter from the constructor of
  WebRtcVideoReceiveChannel and its creation call site.
* Drop now-obsolete virtual overrides and mock definitions across
  fake and mock implementations.

Bug: none
Change-Id: I508393646b4968c808d0c163fc036be969b807e9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/477040
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47886}