Revert "Use just a lookup map of RTP modules in PacketRouter"

This reverts commit 96f3de094566f32d842be6dd0906f1d13b8c8825.

Reason for revert: Downstream test is borked.

Original change's description:
> Use just a lookup map of RTP modules in PacketRouter
> 
> Since SSRCs of RTP modules are now set at construction time, we can
> use just a simple unordered map from SSRC to module in packet router.
> 
> Bug: webrtc:11036
> Change-Id: I0b3527f17c9ee2df9253c778e5b9e3651a70b355
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/155965
> Commit-Queue: Erik Språng <sprang@webrtc.org>
> Reviewed-by: Sebastian Jansson <srte@webrtc.org>
> Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#29510}

TBR=danilchap@webrtc.org,sprang@webrtc.org,srte@webrtc.org

Change-Id: I31330fd68ab809ff3951573791e9a79b81599958
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:11036
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/157281
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29511}

modules/rtp_rtcp/include/rtp_rtcp.h

@@ -226,9 +226,6 @@ class RtpRtcp : public Module, public RtcpFeedbackSenderInterface {
   // a combination of values of the enumerator RtxMode.
   virtual int RtxSendStatus() const = 0;
 
-  // Returns the SSRC used for RTX if set, otherwise a nullopt.
-  virtual absl::optional<uint32_t> RtxSsrc() const = 0;
-
   // Sets the payload type to use when sending RTX packets. Note that this
   // doesn't enable RTX, only the payload type is set.
   virtual void SetRtxSendPayloadType(int payload_type,