Use widget API action to get transports in widget mode

[BillCarsonFr]

There is a new widget action that allows widgets to now call the rtc discovery endpoint:
matrix-org/matrix-spec-proposals#4515

We can now remove the workaround that was not calling _unstable_getRTCTransports to avoid red herring in logs for unauthicated calls.

Depends on element-hq/element-web#34393 - end-to-end test failures are expected until the Element Web PR is merged.

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