nixos/tests/packagekit: fix test machine evaluation

aa22be179a4dfb9633089ebc7c65c6d6c18a83d5 dropped the backend setting
which was used in the test, breaking evaluation of the test in the
process. Kind of defeats the purpose of a test if it isn't executed
before merging a change to a module…

nixos/tests/packagekit.nix

@@ -8,7 +8,6 @@ import ./make-test-python.nix ({ pkgs, ... }: {
     environment.systemPackages = with pkgs; [ dbus ];
     services.packagekit = {
       enable = true;
-      backend = "test_nop";
     };
   };