Remove some obsolete code inside QT_VERSION_CHECK sections

With Qt 5.9 as minimal build version, we don't need to handle 5.7 or 5.8

Change-Id: Ie1eed95ab46364452b0633f78bebc565a4a63f4a
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/help/webenginehelpviewer.h

@@ -46,9 +46,6 @@ class WebEngineHelpPage : public QWebEnginePage
 {
 public:
     explicit WebEngineHelpPage(QObject *parent = nullptr);
-#if QT_VERSION < QT_VERSION_CHECK(5, 7, 0)
-    QWebEnginePage *createWindow(QWebEnginePage::WebWindowType) override;
-#endif
 };
 
 class WebView : public QWebEngineView