Don't pass parameters to wxSizerXmlHandler::GetGB{Pos,Span}()

This doesn't make sense, these functions can only be ever used with a single
parameter ("cellpos" for the former and "cellspan" for the latter), so just
hard-code it inside the functions themselves.

No real changes, just make the code less confusing.

include/wx/xrc/xh_sizer.h

@@ -52,8 +52,8 @@ private:
     bool ValidateGridSizerChildren();
     void SetFlexibleMode(wxFlexGridSizer* fsizer);
     void SetGrowables(wxFlexGridSizer* fsizer, const wxChar* param, bool rows);
-    wxGBPosition GetGBPos(const wxString& param);
-    wxGBSpan GetGBSpan(const wxString& param);
+    wxGBPosition GetGBPos();
+    wxGBSpan GetGBSpan();
     wxSizerItem* MakeSizerItem();
     void SetSizerItemAttributes(wxSizerItem* sitem);
     void AddSizerItem(wxSizerItem* sitem);