nixos: remove genode.init.subinits

This option was never used.

nixos-modules/genode-init.nix

@@ -82,26 +82,6 @@ in {
       description = "Attr set of initial ROM modules";
     };
 
-    subinits =
-      # Subinits are just a different kind of children.
-      # Eventually this will be nested "genode.init" instances.
-      mkOption {
-        default = { };
-        type = with types;
-          attrsOf (submodule {
-            options = {
-              inherit binary coreROMs extraErisInputs extraInputs;
-              configFile = mkOption {
-                type = types.path;
-                description = ''
-                  Dhall configuration of child init.
-                  See https://git.sr.ht/~ehmry/dhall-genode/tree/master/Init/Type
-                '';
-              };
-            };
-          });
-      };
-
   };
 
   config.genode.init = {