Create new instance of wxPrintDC in wxGtkPrintDialog

GetPrintDC() should return a new device context created by the print
dialog, not the duplicate of existing context passed from the caller
(e.g. from wxGtkPrinter) through SetPrintDC(). Therefore SetPrintDC() is
no longer useful and can be removed.

include/wx/gtk/print.h

@@ -80,8 +80,7 @@ public:
     wxPrintDialogData& GetPrintDialogData() wxOVERRIDE
         { return m_printDialogData; }
 
-    wxDC *GetPrintDC() wxOVERRIDE { return m_dc; }
-    void SetPrintDC(wxDC * printDC) { m_dc = printDC; }
+    wxDC *GetPrintDC() wxOVERRIDE;
 
     virtual int ShowModal() wxOVERRIDE;
 
@@ -105,7 +104,6 @@ private:
     wxPrintDialogData    m_printDialogData;
     wxWindow            *m_parent;
     bool                 m_showDialog;
-    wxDC                *m_dc;
 
     wxDECLARE_DYNAMIC_CLASS(wxGtkPrintDialog);
 };