Remove some usage of RtpRtcp::SetSSRC()

Bug: webrtc:10774
Change-Id: Ib8fa84f5d70ceb7e715405eae2901bcd7bdfebfe
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/146984
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28895}

modules/rtp_rtcp/source/rtp_rtcp_impl_unittest.cc

@@ -180,7 +180,6 @@ class RtpRtcpImplTest : public ::testing::Test {
 
   void SetUp() override {
     // Send module.
-    sender_.impl_->SetSSRC(kSenderSsrc);
     EXPECT_EQ(0, sender_.impl_->SetSendingStatus(true));
     sender_.impl_->SetSendingMediaStatus(true);
     sender_.SetRemoteSsrc(kReceiverSsrc);