Make GtkPageSetup-related functions private

These functions don't need to be members of wxGtkPrintNativeData as they
don't use this object at all, so one shouldn't be required to call them.

And rather than making them static, just make them private functions
instead.

No real changes, this is just a refactoring.

include/wx/gtk/print.h

@@ -197,10 +197,6 @@ public:
     GtkPrintContext *GetPrintContext() { return m_context; }
     void SetPrintContext(GtkPrintContext *context) {m_context = context; }
 
-
-    GtkPageSetup* GetPageSetupFromSettings(GtkPrintSettings* settings);
-    void SetPageSetupToSettings(GtkPrintSettings* settings, GtkPageSetup* page_setup);
-
 private:
     // NB: m_config is created and owned by us, but the other objects are not
     //     and their accessors don't change their ref count.