Remove access-token support from RegistrationHandler.register (#5641)

Nothing uses this now, so we can remove the dead code, and clean up the
API.

Since we're changing the shape of the return value anyway, we take the
opportunity to give the method a better name.

synapse/rest/admin/__init__.py

@@ -219,11 +219,10 @@ class UserRegisterServlet(RestServlet):
 
         register = RegisterRestServlet(self.hs)
 
-        (user_id, _) = yield register.registration_handler.register(
+        user_id = yield register.registration_handler.register_user(
             localpart=body["username"].lower(),
             password=body["password"],
             admin=bool(admin),
-            generate_token=False,
             user_type=user_type,
         )