Remove redundant wxDFBDCImpl::m_mm_to_pix_[xy] members

The base class already has exactly the same members, so just use them
instead.

No real changes, just avoid (or at least reduce) confusion.

include/wx/dfb/dc.h

@@ -157,8 +157,6 @@ private:
 protected:
     wxIDirectFBSurfacePtr m_surface;
 
-    double            m_mm_to_pix_x, m_mm_to_pix_y;
-
     friend class WXDLLIMPEXP_FWD_CORE wxOverlayImpl; // for Init
 
     wxDECLARE_ABSTRACT_CLASS(wxDFBDCImpl);