Help: Remove QtWebKit backend

QtWebKit is gone, removing it is better than letting it rot.

Change-Id: Ie0a43f9aed028edb76b2e9f2dac0b4a9be8ce05d
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/help/generalsettingspage.cpp

@@ -46,10 +46,6 @@
 #include <QApplication>
 #include <QFileDialog>
 
-#if !defined(QT_NO_WEBKIT)
-#include <QWebSettings>
-#endif
-
 using namespace Core;
 using namespace Help::Internal;