nixos/grafana-agent: drop server.{grpc,http}_listen_address,http_listen_port

According to https://grafana.com/docs/agent/latest/upgrade-guide/#v0240,
this has been deprecated/moved to -server.http.address and
-server.grpc.address (accepting ip and port) config options in v0.24.0,
and already listens on localhost and not port 80 by default.

nixos/modules/services/monitoring/grafana-agent.nix

@@ -49,13 +49,6 @@ in
       };
 
       default = {
-        server = {
-          # Don't bind on 0.0.0.0
-          grpc_listen_address = "127.0.0.1";
-          http_listen_address = "127.0.0.1";
-          # Don't bind on the default port 80
-          http_listen_port = 9090;
-        };
         metrics = {
           wal_directory = "\${STATE_DIRECTORY}";
           global.scrape_interval = "5s";