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/osx/core/private.h

@@ -615,6 +615,7 @@ public:
 
     virtual void            ListScrollTo( unsigned int n ) = 0;
     virtual int             ListGetTopItem() const = 0;
+    virtual int             ListGetCountPerPage() const = 0;
     virtual void            UpdateLine( unsigned int n, wxListWidgetColumn* col = NULL ) = 0;
     virtual void            UpdateLineToEnd( unsigned int n) = 0;