nixos/gdm: remove udev-settle dependency

See NixOS#73095

It looks like it can finally be removed: the `Failed to open gpu
'/dev/dri/card0'` error is gone and the nixosTests.gnome test is
passing (checked 4 times in a row).

nixos/modules/services/x11/display-managers/gdm.nix

@@ -174,9 +174,6 @@ in
       "systemd-machined.service"
       # setSessionScript wants AccountsService
       "accounts-daemon.service"
-      # Failed to open gpu '/dev/dri/card0': GDBus.Error:org.freedesktop.DBus.Error.AccessDenied: Operation not permitted
-      # https://github.com/NixOS/nixpkgs/pull/25311#issuecomment-609417621
-      "systemd-udev-settle.service"
     ];
 
     systemd.services.display-manager.after = [
@@ -186,7 +183,6 @@ in
       "getty@tty${gdm.initialVT}.service"
       "plymouth-quit.service"
       "plymouth-start.service"
-      "systemd-udev-settle.service"
     ];
     systemd.services.display-manager.conflicts = [
       "getty@tty${gdm.initialVT}.service"