Do not rotate MatrixRTC media encryption key when a new member joins a session (#4472)
* Do not rotate MatrixRTC media encryption key when a new member joins a call
This change reverts https://github.com/matrix-org/matrix-js-sdk/pull/4422.
Instead, the rotation when a new member joins will be reintroduced as part of supporting to-device based MatrixRTC encryption key distribution.
* Improve function name