Show UBL Activate/Deactivate menu for current state (#13744)

Marlin/src/lcd/menu/menu_ubl.cpp

 void _lcd_ubl_level_bed() {
   START_MENU();
   MENU_BACK(MSG_MOTION);
-  MENU_ITEM(gcode, MSG_UBL_ACTIVATE_MESH, PSTR("G29 A"));
-  MENU_ITEM(gcode, MSG_UBL_DEACTIVATE_MESH, PSTR("G29 D"));
+  if (planner.leveling_active)
+    MENU_ITEM(gcode, MSG_UBL_DEACTIVATE_MESH, PSTR("G29 D"));
+  else
+    MENU_ITEM(gcode, MSG_UBL_ACTIVATE_MESH, PSTR("G29 A"));
   MENU_ITEM(submenu, MSG_UBL_STEP_BY_STEP_MENU, _lcd_ubl_step_by_step);
   MENU_ITEM(function, MSG_UBL_MESH_EDIT, _lcd_ubl_output_map_lcd_cmd);
   MENU_ITEM(submenu, MSG_UBL_STORAGE_MESH_MENU, _lcd_ubl_storage_mesh);