Re: [webrtc-pc] Remove event.receiver.

I think in hindsight I'm withdrawing this PR. More browsers than I realized already support this, i.e. this works in Safari:
```js
pc.ontrack = event => {
  console.log(event.track);
  console.log(event.receiver.track);
  console.log(event.transceiver.receiver.track);
  video.srcObject = event.streams[0];
};
```
The redundancy extends to `track` as well, a can of worms best left unopened, and the value of removing any of these seem marginal, whereas the compat hit substantial. 

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