Refactor/simplify Promises in MatrixRTCSession

[AndrewFerr]

Signed-off-by: Andrew Ferrazzutti [email protected]

Checklist

• [ ] Tests written for new code (and old code if feasible).
• [ ] New or updated public/exported symbols have accurate TSDoc documentation.
• [ ] Linter and other CI checks pass.
• [x] Sign-off given on the changes (see CONTRIBUTING.md).