Remove RTCIdentityEvent and RTCIdentityErrorEvent

These were only ever implemented in Firefox and removed in 2015:
https://bugzilla.mozilla.org/show_bug.cgi?id=975144
mozilla/gecko-dev@e92b44d

The events identityresult, peeridentity, idpassertionerror and
idpvalidationerror were also removed then.

The data claiming support in Edge 18 for some of this is wrong,
confirmed by testing Edge 18 with these tests:
https://mdn-bcd-collector.appspot.com/tests/api/RTCPeerConnection/onidentityresult
https://mdn-bcd-collector.appspot.com/tests/api/RTCPeerConnection/onidpassertionerror
https://mdn-bcd-collector.appspot.com/tests/api/RTCPeerConnection/onidpvalidationerror
https://mdn-bcd-collector.appspot.com/tests/api/RTCPeerConnection/onpeeridentity