Re: [webrtc-pc] End removed tracks remotely again; Make receiver.track nullable instead

I think it would be good to work through the events (in terms of `pc.removeTrack`, `transceiver.setDirection` and so on, assuming `onnegotiationneeded` is wired up) that would lead to "send" and "recv" bits being set/unset.

But overall this looks like something that could work. If the intent is to have the received track being constantly played or recorded, the sender can play with `replaceTrack` and use a null track for periods when there is nothing to send.

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