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/include/rtp_rtcp.h

@@ -39,6 +39,14 @@ class RtpRtcp : public Module, public RtpRtcpInterface {
       RTPExtensionType type,
       uint8_t id) = 0;
 
+  // (APP) Sets application specific data.
+  // Returns -1 on failure else 0.
+  RTC_DEPRECATED virtual int32_t SetRTCPApplicationSpecificData(
+      uint8_t sub_type,
+      uint32_t name,
+      const uint8_t* data,
+      uint16_t length) = 0;
+
   // Requests new key frame.
   // using PLI, https://tools.ietf.org/html/rfc4585#section-6.3.1.1
   void SendPictureLossIndication() { SendRTCP(kRtcpPli); }