Remove dead code "< QT_VERSION_CHECK(5, 14, 0)"

Qt Creator required Qt 5.14 or higher. Let's removed the #idef-ed code
for building against lower Qt versions.

Add a comment in porting.h where we still need such code for sdktool.

Change-Id: Ib330275208eec4a2f285b4b9f480530c171f538a
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/scxmleditor/plugin_interface/sceneutils.cpp

@@ -225,11 +225,7 @@ void layout(const QList<QGraphicsItem*> &items)
         firstItem = initialItem->outputTransitions().constFirst()->connectedItem(initialItem);
         int index = childItems.indexOf(firstItem);
         if (index > 0)
-#if QT_VERSION < QT_VERSION_CHECK(5, 13, 0)
-            childItems.swap(index, 0);
-#else
             childItems.swapItemsAt(index, 0);
-#endif
     }
 
     // Search final-item