Remove redundant `get_current_events_token` (#11643)

* Push `get_room_{min,max_stream_ordering}` into StreamStore

Both implementations of this are identical, so we may as well push it down and
get rid of the abstract base class nonsense.

* Remove redundant `StreamStore` class

This is empty now

* Remove redundant `get_current_events_token`

This was an exact duplicate of `get_room_max_stream_ordering`, so let's get rid
of it.

* newsfile