Improve wxListBox::GetCountPerPage() in wxGTK and wxOSX

Provide native implementation of this function instead of using the ad hoc one
in common code, which didn't really work -- so remove it completely now.

Closes #17189.

include/wx/gtk/listbox.h

@@ -75,6 +75,7 @@ public:
     virtual void EnsureVisible(int n) wxOVERRIDE;
 
     virtual int GetTopItem() const wxOVERRIDE;
+    virtual int GetCountPerPage() const wxOVERRIDE;
 
     virtual void Update() wxOVERRIDE;