Remove unused instance tree related member variable

Source/core/page/EventHandler.h

 /*
  * Copyright (C) 2006, 2007, 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 316 matching lines @@
 
     Timer<EventHandler> m_hoverTimer;
     Timer<EventHandler> m_cursorUpdateTimer;
 
     bool m_mouseDownMayStartAutoscroll;
     bool m_mouseDownWasInSubframe;
 
     Timer<EventHandler> m_fakeMouseMoveEventTimer;
 
     bool m_svgPan;
-    RefPtr<SVGElementInstance> m_instanceUnderMouse;
     RefPtr<SVGElementInstance> m_lastInstanceUnderMouse;
 
     RenderLayerScrollableArea* m_resizeScrollableArea;
 
     RefPtr<Node> m_capturingMouseEventsNode;
     bool m_eventHandlerWillResetCapturingMouseEventsNode;
 
     RefPtr<Node> m_nodeUnderMouse;
     RefPtr<Node> m_lastNodeUnderMouse;
     RefPtr<LocalFrame> m_lastMouseMoveEventSubframe;
@@ skipping 41 matching lines @@
     bool m_longTapShouldInvokeContextMenu;
 
     Timer<EventHandler> m_activeIntervalTimer;
     double m_lastShowPressTimestamp;
     RefPtr<Element> m_lastDeferredTapElement;
 };
 
 } // namespace WebCore
 
 #endif // EventHandler_h