Do not save the settings twice

Settings are saved in MainWindow::closeEvent() which is called when Sdrangel
is exited by closing its main window, or when using the File/Exit menu (or
Ctrl+Q) or when pressing Alt+F4.

sdrgui/mainwindow.cpp

@@ -319,7 +319,6 @@ MainWindow::~MainWindow()
 	qDebug() << "MainWindow::~MainWindow";
 
     m_statusTimer.stop();
-    m_mainCore->m_settings.save();
     m_apiServer->stop();
     delete m_apiServer;
     delete m_requestMapper;