Revert "new systemd can't inhibit lid switches"

This reverts commit f9fb7b7457.
Cf. https://github.com/jcumming/nixos/commit/da2c160471f73af

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

@@ -39,6 +39,16 @@ let
         exec > ~/.xsession-errors 2>&1
       ''}
 
+      ${optionalString cfg.displayManager.desktopManagerHandlesLidAndPower ''
+        # Stop systemd from handling the power button and lid switch,
+        # since presumably the desktop environment will handle these.
+        if [ -z "$_INHIBITION_LOCK_TAKEN" ]; then
+          export _INHIBITION_LOCK_TAKEN=1
+          exec ${config.systemd.package}/bin/systemd-inhibit --what=handle-lid-switch:handle-power-key "$0" "$sessionType"
+        fi
+
+      ''}
+
       ${optionalString cfg.startOpenSSHAgent ''
         if test -z "$SSH_AUTH_SOCK"; then
             # Restart this script as a child of the SSH agent.  (It is