Fix 3PID invite room state over federation.

Fixes that when a user exchanges a 3PID invite for a proper invite over
federation it does not include the `invite_room_state` key.

This was due to synapse incorrectly sending out two invite requests.

synapse/handlers/federation.py

@@ -2613,12 +2613,6 @@ class FederationHandler(BaseHandler):
         # though the sender isn't a local user.
         event.internal_metadata.send_on_behalf_of = get_domain_from_id(event.sender)
 
-        # XXX we send the invite here, but send_membership_event also sends it,
-        # so we end up making two requests. I think this is redundant.
-        returned_invite = yield self.send_invite(origin, event)
-        # TODO: Make sure the signatures actually are correct.
-        event.signatures.update(returned_invite.signatures)
-
         member_handler = self.hs.get_room_member_handler()
         yield member_handler.send_membership_event(None, event, context)