message: remove unused SetCnameIfEmpty
  Cr-Commit-Position: refs/heads/main@{#37175}
diff: @@ -183,14 +183,6 @@ class MediaContentDescription {
  AddStream(sp);
  }
  - // Sets the CNAME of all StreamParams if it have not been set.
  - virtual void SetCnameIfEmpty(const std::string& cname) {
  - for (cricket::StreamParamsVec::iterator it = send_streams_.begin();
  - it != send_streams_.end(); ++it) {
  - if (it->cname.empty())
  - it->cname = cname;
  - }
  - }
  virtual uint32_t first_ssrc() const {
  if (send_streams_.empty()) {
  return 0;
  