Revert "Add ability to set RTCP sender ssrc at construction time"

This reverts commit 94c58fd815f0c7c6429aa53a79621ea9ef39c770.

Reason for revert: Speculative revert, as it looks like this one broke IOS debug perf bots: https://ci.chromium.org/p/webrtc-internal/builders/ci/iOS64%20Debug/18901

Original change's description:
> Add ability to set RTCP sender ssrc at construction time
> 
> Bug: webrtc:10774
> Change-Id: Iaf5857e24359e9795434bcd0cdbe1658a2f9f5d3
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/144632
> Reviewed-by: Åsa Persson <asapersson@webrtc.org>
> Commit-Queue: Erik Språng <sprang@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#28506}

TBR=asapersson@webrtc.org,sprang@webrtc.org

Change-Id: I3f377ca1c84a7448675e5d022cb2f86f9630dbaf
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:10774
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/144564
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28508}

modules/rtp_rtcp/source/rtp_rtcp_impl_unittest.cc

@@ -162,7 +162,6 @@ class RtpRtcpModule : public RtcpPacketTypeCounterObserver {
     config.rtcp_packet_type_counter_observer = this;
     config.rtt_stats = &rtt_stats_;
     config.rtcp_report_interval_ms = rtcp_report_interval_ms_;
-    config.media_send_ssrc = kSenderSsrc;
 
     impl_.reset(new ModuleRtpRtcpImpl(config));
     impl_->SetRTCPStatus(RtcpMode::kCompound);