Remove unused method in synapse.api.auth.Auth. (#13795)

Clean-up from b19060a (#13094)
and 73af10f (#13093) which removed
all callers.

changelog.d/13795.misc

@@ -0,0 +1 @@
+Remove unused method in `synapse.api.auth.Auth`.

synapse/api/auth.py

@@ -459,15 +459,6 @@ async def get_user_by_access_token(
             )
             raise InvalidClientTokenError("Invalid access token passed.")
 
-    def get_appservice_by_req(self, request: SynapseRequest) -> ApplicationService:
-        token = self.get_access_token_from_request(request)
-        service = self.store.get_app_service_by_token(token)
-        if not service:
-            logger.warning("Unrecognised appservice access token.")
-            raise InvalidClientTokenError()
-        request.requester = create_requester(service.sender, app_service=service)
-        return service
-
     async def is_server_admin(self, requester: Requester) -> bool:
         """Check if the given user is a local server admin.