fix(rdp): run grdctl --system directly as root, remove runuser/polkit/util-linux

Root cause: gnome-remote-desktop-setup.service ran as root but dropped
privileges via `runuser -u gnome-remote-desktop`. Non-root grdctl --system
attempts authorisation through pkexec. The Nix-store pkexec binary is not
setuid, so every system-mode credential call silently failed while the script
still printed "configured successfully". GNOME Remote Desktop then started
without applied credentials, causing Remmina to loop at the login dialog.

Fix:
- Remove `runuser -u gnome-remote-desktop --` from grdctl_system helper;
  the root-run oneshot service can call grdctl --system directly.
- Remove pkgs.polkit and pkgs.util-linux from the setup service path as
  neither polkit nor runuser is needed any more.
- Update tests: rename test_setup_runs_grdctl_as_gnome_remote_desktop_user
  to test_setup_runs_grdctl_directly_as_root and assert that runuser,
  pkexec, and sudo are absent; add
  test_privilege_escalation_packages_absent_from_setup_path.

All 21 app/tests pass.

modules/rdp.nix

@@ -46,9 +46,7 @@ lib.mkIf config.sovran_systemsOS.features.rdp {
       pkgs.gnome-remote-desktop
       pkgs.hostname
       pkgs.openssl
-      pkgs.polkit
       pkgs.systemd
-      pkgs.util-linux
     ];
     script = ''
       set -euo pipefail
@@ -64,7 +62,6 @@ lib.mkIf config.sovran_systemsOS.features.rdp {
         local rc=0
 
         if timeout --kill-after=5s 10s \
-          runuser -u gnome-remote-desktop -- \
           grdctl --system "$@"; then
           return 0
         else