tests: Remove desktop import

Fixes an issue where NixOS tests weren't exactly treated as
containers, presumably since they run inside QEMU.

The previous dependency on stylix is no longer necessary since
base16-nvim was upstreamed, although it may be ideal to split
it into its own module at a later date.

tests/neovim.nix

@@ -9,7 +9,6 @@ self.inputs.nixpkgs.lib.nixos.runTest {
     { nix-config, ... }:
     {
       imports = with nix-config.nixosModules; [
-        desktop
         shell
         system
       ];