nixos/profiles/minimal: don't install freedesktop files

This saves about 25M from the closure size of:

$ nix build -f nixos system --arg configuration '
  { imports = [ ./nixos/modules/profiles/minimal.nix ];
    fileSystems."/".label="root";
    boot.loader.grub.device = "nodev";
  }'

nixos/modules/profiles/minimal.nix

@@ -13,4 +13,9 @@ with lib;
   documentation.nixos.enable = mkDefault false;
 
   programs.command-not-found.enable = mkDefault false;
+
+  xdg.autostart.enable = mkDefault false;
+  xdg.icons.enable = mkDefault false;
+  xdg.mime.enable = mkDefault false;
+  xdg.sounds.enable = mkDefault false;
 }