remove HomeServer.get_config (#9815)

Every single time I want to access the config object, I have to remember
whether or not we use `get_config`. Let's just get rid of it.

synapse/app/generic_worker.py

@@ -405,7 +405,7 @@ class GenericWorkerServer(HomeServer):
                     listener.bind_addresses, listener.port, manhole_globals={"hs": self}
                 )
             elif listener.type == "metrics":
-                if not self.get_config().enable_metrics:
+                if not self.config.enable_metrics:
                     logger.warning(
                         (
                             "Metrics listener configured, but "