[QT] some mac specifiy cleanup (memory handling, unnecessary code)

src/qt/macdockiconhandler.mm

@@ -68,7 +68,6 @@ void MacDockIconHandler::setMainWindow(QMainWindow *window) {
 
 MacDockIconHandler::~MacDockIconHandler()
 {
-    [this->m_dockIconClickEventHandler release];
     delete this->m_dummyWidget;
     this->setMainWindow(NULL);
 }
@@ -118,6 +117,11 @@ MacDockIconHandler *MacDockIconHandler::instance()
     return s_instance;
 }
 
+void MacDockIconHandler::cleanup()
+{
+    delete s_instance;
+}
+
 void MacDockIconHandler::handleDockIconClickEvent()
 {
     if (this->mainWindow)