nixosTests.flatpak: clean up

GNOME is not necessary. Portals probably are not either,
but the NixOS module requires them.

Not sure why it did not work without GNOME before.

nixos/tests/installed-tests/flatpak.nix

@@ -5,10 +5,7 @@ makeInstalledTest {
   withX11 = true;
 
   testConfig = {
-    services.xserver.desktopManager.gnome3.enable = true; # TODO: figure out minimal environment where the tests work
-    # common/x11.nix enables the auto display manager (lightdm)
-    services.xserver.displayManager.gdm.enable = false;
-    services.gnome3.core-utilities.enable = false;
+    xdg.portal.enable = true;
     services.flatpak.enable = true;
     environment.systemPackages = with pkgs; [ gnupg ostree python3 ];
     virtualisation.memorySize = 2047;