The spec says that users must forget a room on their own, however I'm not really aware of many people who do this intentionally. To assist homeservers in their ability to garbage collect rooms, maybe we should allow them to forget rooms on a user's behalf.

We might want to put restrictions on it (ie: "thou shall not forget a room which has been left in the last 15 days"), but homeservers should get the ability to automatically and arbitrarily forget rooms.