Signed-off-by: Jonathan Frederickson <jonathan@terracrypt.net>

Based on feedback in PR discussion; `room_type` is redundant given the
context.

Just for cleanliness here, since domain.com is a real thing.

I'm unclear on whether this should be done by the client on room
creation or by the server as part of `createRoom`. Open to feedback
here.

Initially I was considering the default to be `m.messaging`, but
@julianfoad brings up a good point that this isn't necessarily a good
assumption. I'm inclined to leave the decision of what to do with
untyped rooms up to the clients, while recommending that they set room
types for new rooms going forward.