examples/qt_opengl: remove redundant time-start property fetch

Time-start is not a real property since 70df1608d.

Fixes #2529.

DOCS/client_api_examples/qt_opengl/mainwindow.cpp

@@ -48,6 +48,5 @@ void MainWindow::pauseResume()
 
 void MainWindow::setSliderRange(int duration)
 {
-    const int time0 = m_mpv->getProperty("time-start").toInt();
-    m_slider->setRange(time0, time0+duration);
+    m_slider->setRange(0, duration);
 }