Remove registered_users from the distributor.

The only place that was observed was to set the profile. I've made it
so that the profile is set within store.register in the same transaction
that creates the user.

This required some slight changes to the registration code for upgrading
guest users, since it previously relied on the distributor swallowing errors
if the profile already existed.

synapse/handlers/profile.py

@@ -36,13 +36,6 @@ class ProfileHandler(BaseHandler):
             "profile", self.on_profile_query
         )
 
-        distributor = hs.get_distributor()
-
-        distributor.observe("registered_user", self.registered_user)
-
-    def registered_user(self, user):
-        return self.store.create_profile(user.localpart)
-
     @defer.inlineCallbacks
     def get_displayname(self, target_user):
         if self.hs.is_mine(target_user):