nixosTests.flatpak: clean up
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/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs