macOS: Remove old friend declaration for qt_mac_is_metal

The function was part of the Cocoa style for checking the metal
look (not Metal the graphics API), but has long been removed.

Pick-to: 6.5 6.6
Change-Id: I366b952db4ae82b8ecc442f1ce61e7f53cacfe80
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/widgets/kernel/qwidget.h

@@ -769,9 +769,6 @@ private:
 #endif // QT_NO_GESTURES
     friend class QWidgetEffectSourcePrivate;
 
-#ifdef Q_OS_MAC
-    friend bool qt_mac_is_metal(const QWidget *w);
-#endif
     friend Q_WIDGETS_EXPORT QWidgetData *qt_qwidget_data(QWidget *widget);
     friend Q_WIDGETS_EXPORT QWidgetPrivate *qt_widget_private(QWidget *widget);