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/listbox.h

@@ -73,7 +73,7 @@ public:
     virtual void EnsureVisible(int n);
 
     virtual int GetTopItem() const { return wxNOT_FOUND; }
-    virtual int GetCountPerPage() const;
+    virtual int GetCountPerPage() const { return -1; }
 
     // a combination of Append() and EnsureVisible(): appends the item to the
     // listbox and ensures that it is visible i.e. not scrolled out of view
@@ -136,8 +136,6 @@ protected:
     // single selection mode on platforms other than MSW).
     void UpdateOldSelections();
 
-    wxCoord GetLineHeight() const;
-
 private:
     wxDECLARE_NO_COPY_CLASS(wxListBoxBase);
 };