Remove wxGTKCairoDCImpl ctor taking dummy int

We can reuse another ctor taking wxWindow* instead. Although this does
require an ugly cast, it's more explicit than just passing 0 which
could, in principle, match both the ctor taking wxWindow* and another
one taking double, and so is still preferable.

include/wx/gtk/dc.h

@@ -17,7 +17,6 @@ class wxGTKCairoDCImpl: public wxGCDCImpl
 {
 public:
     wxGTKCairoDCImpl(wxDC* owner);
-    wxGTKCairoDCImpl(wxDC* owner, int);
     wxGTKCairoDCImpl(wxDC* owner, double scaleFactor);
     wxGTKCairoDCImpl(wxDC* owner, wxWindow* window);