Remove SetRTCPApplicationSpecificData.

Also removing some related code that appears to be unused.
This is a part of simplifying the RtpRtcpInterface implementation.

Bug: webrtc:11581
Change-Id: I580bfdc1b821d571cb7437d7713a49ee4de2d19a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/176568
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31464}

modules/rtp_rtcp/source/rtp_rtcp_impl2.h

@@ -246,12 +246,6 @@ class ModuleRtpRtcpImpl2 final : public RtpRtcpInterface,
   void SendCombinedRtcpPacket(
       std::vector<std::unique_ptr<rtcp::RtcpPacket>> rtcp_packets) override;
 
-  // (APP) Application specific data.
-  int32_t SetRTCPApplicationSpecificData(uint8_t sub_type,
-                                         uint32_t name,
-                                         const uint8_t* data,
-                                         uint16_t length) override;
-
   // (XR) Receiver reference time report.
   void SetRtcpXrRrtrStatus(bool enable) override;