Remove Carbon-specific code from wxScreenDC implementation

Carbon is not supported any more and defining m_overlayWindow, which was
only used by Carbon code, resulted in "unused private variable" warning
from clang.

include/wx/osx/dcscreen.h

@@ -21,8 +21,6 @@ public:
     virtual ~wxScreenDCImpl();
 
     virtual wxBitmap DoGetAsBitmap(const wxRect *subrect) const wxOVERRIDE;
-private:
-    void* m_overlayWindow;
 
 private:
     wxDECLARE_CLASS(wxScreenDCImpl);