webrtcbin: Remove non-rtcpmux mode

While working on the code to configure the buffer sizes, I couldn't understand what "rtcp_transport" was supposed to match in the WebRTC javascript spec. I finally realized that it had been removed when they decided to make rtcp-mux mandatory. So I think we should do the same, it also simplified the code quite a bit. We can drop the selector code, etc. And in any case, rtcp-mux was hardcoded to TRUE inside webrtcbin anyway.

I'm removing rtcp_transport from the structure. In any case, I intend to submit a follow up patch to hide the entire structure from the public API as they're unsafe. I'm actually wondering if we shouldn't make RTPSender/RTPReceiver/DTLSTransport into interfaces instead as they hold no code.

There is again a gstreamer-sharp!28 (merged) companion MR.