Remove Apple's unused implementation of private browsing from WebCore

Source/WebCore/dom/Element.h

Index: Source/WebCore/dom/Element.h
diff --git a/Source/WebCore/dom/Element.h b/Source/WebCore/dom/Element.h
index beb0c9fd4564a9824cb72d8541b3b905efb063fb..ba20b1fc935af3b037c1a7152b7e178b6e9988dc 100644
--- a/Source/WebCore/dom/Element.h
+++ b/Source/WebCore/dom/Element.h
@@ -483,9 +483,6 @@ public:
     // Use Document::registerForMediaVolumeCallbacks() to subscribe to this
     virtual void mediaVolumeDidChange() { }
 
-    // Use Document::registerForPrivateBrowsingStateChangedCallbacks() to subscribe to this.
-    virtual void privateBrowsingStateDidChange() { }
-
     virtual void didBecomeFullscreenElement() { }
     virtual void willStopBeingFullscreenElement() { }