When a server leaves a room it may stop sharing a room with remote
users, and thus not get any updates to their device lists. So we need to
check for this case and delete those device lists from the cache.
We don't need to do this if we stop sharing a room because the remote
user leaves the room, because we track that case via looking at
membership changes.
Otherwise its just stale data, which may get deleted later anyway so
can't be relied on. It's also a bit of a shotgun if we're trying to get
the current state of a room we're not in.
