Remove call to Quit from backend.quit method

the backend.quit emits the 'quitDone' signal which also triggers the 'aboutToQuit' signal for the QApplication since its handler calls 'QApplication::quit()'

the handler for 'aboutToQuit' signal already calls Quit so removing it from backend.quit avoids another call to Quit

gui/handlers.cpp

@@ -99,7 +99,6 @@ void Backend::resetNotification(QString label)
 
 void Backend::quit()
 {
-    Quit();
     emit this->quitDone();
 }