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/projectexplorer/buildsteplist.cpp

@@ -192,11 +192,7 @@ bool BuildStepList::removeStep(int position)
 
 void BuildStepList::moveStepUp(int position)
 {
-#if QT_VERSION >= QT_VERSION_CHECK(5, 13, 0)
     m_steps.swapItemsAt(position - 1, position);
-#else
-    m_steps.swap(position - 1, position);
-#endif
     emit stepMoved(position, position - 1);
 }