nixos/nh: allow independent nh clean service

Author: musjj

nixos/modules/programs/nh.nix

@@ -76,12 +76,6 @@ in
         [ ];
 
     assertions = [
-      # Not strictly required but probably a good assertion to have
-      {
-        assertion = cfg.clean.enable -> cfg.enable;
-        message = "programs.nh.clean.enable requires programs.nh.enable";
-      }
-
       {
         assertion = (cfg.flake != null) -> !(lib.hasSuffix ".nix" cfg.flake);
         message = "nh.flake must be a directory, not a nix file";