Remove dead code

Leftover from the system tray code refactoring.

src/gui/mainwindow.cpp

@@ -50,10 +50,6 @@
 #include <QtGlobal>
 #include <QTimer>
 
-#ifdef QBT_USES_CUSTOMDBUSNOTIFICATIONS
-#include "notifications/dbusnotifier.h"
-#endif
-
 #include "base/bittorrent/session.h"
 #include "base/bittorrent/sessionstatus.h"
 #include "base/global.h"

src/gui/mainwindow.h

@@ -68,11 +68,6 @@ namespace Ui
     class MainWindow;
 }
 
-#if (defined(Q_OS_UNIX) && !defined(Q_OS_MACOS)) && defined(QT_DBUS_LIB)
-#define QBT_USES_CUSTOMDBUSNOTIFICATIONS
-class DBusNotifier;
-#endif
-
 class MainWindow final : public QMainWindow, public GUIApplicationComponent
 {
     Q_OBJECT