When a remote user leaves the last room shared with the homeserver, we
have to mark their device list as unsubscribed, otherwise we will hold
on to a stale device list in our cache. Crucially, the device list will
remain cached even after the remote user rejoins the room, which can
lead to E2EE failures until the remote user's device list next changes.

Signed-off-by: Sean Quah <seanq@matrix.org>

Now that we are handling device list unsubscriptions in the event
persistence code, it's no longer necessary to mark device lists as
unsubscribed elsewhere.

Signed-off-by: Sean Quah <seanq@matrix.org>

…racking