[GUI] Remove code for Qt < 5.9

src/Gui/GuiApplication.cpp

@@ -63,14 +63,9 @@ using namespace Gui;
 GUIApplication::GUIApplication(int & argc, char ** argv)
     : GUIApplicationNativeEventAware(argc, argv)
 {
-#if QT_VERSION > 0x050000
-    // In Qt 4.x 'commitData' is a virtual method
     connect(this, SIGNAL(commitDataRequest(QSessionManager &)),
             SLOT(commitData(QSessionManager &)), Qt::DirectConnection);
-#endif
-#if QT_VERSION >= 0x050600
     setFallbackSessionManagementEnabled(false);
-#endif
 }
 
 GUIApplication::~GUIApplication()