Optimise preheat menu's MENU_ITEM_BACK_P

Saves 10 bytes of flash memory

Firmware/ultralcd.cpp

@@ -2400,14 +2400,7 @@ void lcd_generic_preheat_menu()
         ON_MENU_LEAVE(
             mFilamentBack();
         );
-        if (eFilamentAction == FilamentAction::Lay1Cal)
-        {
-            MENU_ITEM_BACK_P(_T(MSG_BACK));
-        }
-        else
-        {
-            MENU_ITEM_BACK_P(_T(MSG_MAIN));
-        }
+        MENU_ITEM_BACK_P(_T(eFilamentAction == FilamentAction::Lay1Cal ? MSG_BACK : MSG_MAIN));
     }
     if (farm_mode)
     {