RTCPeerConnection leak

[dbkr]

Describe the bug
We appear to be leaking RTCPeerConnections between calls

To Reproduce
Steps to reproduce the behavior:

1. Enter a group call
2. Leave
3. Maybe repeat above steps a few times to make it more obvious
4. Take heap snapshot
5. Observe RTCPeerConnection(s) still referenced

[robintown]

Fixed by matrix-org/matrix-js-sdk#2484