Revert "nixos/test-instrumentation: softcode DISPLAY"

This reverts commit 4531ec5955.

This broke waitForX: https://github.com/NixOS/nixpkgs/commit/b11ac5d29dcccdb7474a23ac1d8a6c40553adcd5#commitcomment-25486347

cc @Lassulus

nixos/modules/testing/test-instrumentation.nix

@@ -5,10 +5,9 @@
 
 with lib;
 
-let
-  display = (d: if d == null then 0 else d) config.services.xserver.display;
-  kernel = config.boot.kernelPackages.kernel;
-in {
+let kernel = config.boot.kernelPackages.kernel; in
+
+{
 
   # This option is a dummy that if used in conjunction with
   # modules/virtualisation/qemu-vm.nix gets merged with the same option defined
@@ -29,7 +28,7 @@ in {
           ''
             export USER=root
             export HOME=/root
-            export DISPLAY=:${toString display}
+            export DISPLAY=:0.0
 
             source /etc/profile