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

enabled an automatic merge when the pipeline for bd2b8dcd succeeds

merged