Remove obj_c for macOS Dock icon setting

Qt `setWindowIcon()` does this work.

src/qt/bitcoingui.cpp

@@ -92,12 +92,8 @@ BitcoinGUI::BitcoinGUI(interfaces::Node& node, const PlatformStyle *_platformSty
         windowTitle += tr("Node");
     }
     windowTitle += " " + networkStyle->getTitleAddText();
-#ifndef Q_OS_MAC
     QApplication::setWindowIcon(networkStyle->getTrayAndWindowIcon());
     setWindowIcon(networkStyle->getTrayAndWindowIcon());
-#else
-    MacDockIconHandler::instance()->setIcon(networkStyle->getAppIcon());
-#endif
     setWindowTitle(windowTitle);
 
     rpcConsole = new RPCConsole(node, _platformStyle, 0);