feat(meta): Makes it mandatory to declare gateways for an interface

The list might still be empty when using DHCP

meta/options.nix

@@ -169,7 +169,6 @@ in
 
                         gateways = mkOption {
                           type = listOf str;
-                          default = [ ];
                           description = ''
                             List of gateways used by the interface.
                           '';
@@ -251,6 +250,4 @@ in
       '';
     };
   };
-
-  config = { };
 }