Replace virtual RtcpPacket::SetSenderSsrc with base member

to slightly improve binary size.

Bug: None
Change-Id: I894c7d67a72f4a8077963d2ba0a7bb471a2e7e4d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/156300
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29428}

modules/rtp_rtcp/source/rtcp_packet/compound_packet.h

@@ -35,8 +35,6 @@ class CompoundPacket : public RtcpPacket {
               size_t max_length,
               PacketReadyCallback callback) const override;
 
-  void SetSenderSsrc(uint32_t /*ssrc*/) override { RTC_NOTREACHED(); }
-
  protected:
   std::vector<RtcpPacket*> appended_packets_;