ui: remove unused single monitor visibility rule

src/ui/common/displays-visibility.vala

@@ -83,8 +83,6 @@ namespace TunerDisplays {
                 || controller.mirror_mode_visible();
         case "details":
             return !controller.mirror_enabled() && !controller.single_monitor_mode();
-        case "single-monitor":
-            return controller.single_monitor_mode();
         case "virtual-outputs":
             return controller.backend.supports_virtual_outputs;
         default: