Removes TransportSequenceNumberAllocator

This interface/config field is now unused, let's remove it.

Bug: webrtc:10633
Change-Id: I56ff3d47ba784d973de411ada52ec9485bad9864
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150531
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28978}

modules/rtp_rtcp/include/rtp_rtcp_defines.h

@@ -343,14 +343,6 @@ class RtcpRttStats {
   virtual ~RtcpRttStats() {}
 };
 
-class TransportSequenceNumberAllocator {
- public:
-  TransportSequenceNumberAllocator() {}
-  virtual ~TransportSequenceNumberAllocator() {}
-
-  virtual uint16_t AllocateSequenceNumber() = 0;
-};
-
 struct RtpPacketCounter {
   RtpPacketCounter()
       : header_bytes(0), payload_bytes(0), padding_bytes(0), packets(0) {}