Remove obsolete RoomEventsStoreTestCase (#13200)

All tests are prefixed with `STALE_` and therefore they are silently
skipped. They were moved to `STALE_` in version `v0.5.0` in commit
2fcce3b3 - `Remove stale tests`.

Tests from `RoomEventsStoreTestCase` class are not used for last 8
years, I believe the best would be to remove them entirely.

Signed-off-by: Petr Vaněk <arkamar@atlas.cz>