Hook to remove empty rooms

[jcabannes]

Rooms should be remove when the last member left it OR when the room only contains users from an external matrix server

[pm-McFly]

This is configured directly on synapse:

https://github.com/element-hq/synapse/blob/develop/docs/usage/configuration/config_documentation.md#forget_rooms_on_leave

closing as it is already available and config per matrix instance