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/qmlprofiler/qmlprofilertraceview.cpp

@@ -295,12 +295,8 @@ void QmlProfilerTraceView::showContextMenu(QPoint position)
 
 bool QmlProfilerTraceView::isUsable() const
 {
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 8, 0))
     return d->m_mainView->quickWindow()->rendererInterface()->graphicsApi()
             == QSGRendererInterface::OpenGL;
-#else
-    return true;
-#endif
 }
 
 bool QmlProfilerTraceView::isSuspended() const