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.

src/common/lboxcmn.cpp

@@ -34,15 +34,6 @@
     #include "wx/dcclient.h"
 #endif
 
-// the spacing between the lines (in report mode)
-static const int LINE_SPACING = 0;
-
-#ifdef __WXGTK__
-static const int EXTRA_HEIGHT = 6;
-#else
-static const int EXTRA_HEIGHT = 4;
-#endif
-
 extern WXDLLEXPORT_DATA(const char) wxListBoxNameStr[] = "listBox";
 
 // ============================================================================
@@ -349,24 +340,4 @@ void wxListBoxBase::EnsureVisible(int WXUNUSED(n))
     // the base class version does nothing (the only alternative would be to
     // call SetFirstItem() but this is probably even more stupid)
 }
-
-wxCoord wxListBoxBase::GetLineHeight() const
-{
-    wxListBoxBase *self = wxConstCast(this, wxListBoxBase);
-
-    wxClientDC dc( self );
-    dc.SetFont( GetFont() );
-
-    wxCoord y;
-    dc.GetTextExtent(wxT("H"), NULL, &y);
-
-    y += EXTRA_HEIGHT;
-
-    return y + LINE_SPACING;
-}
-
-int wxListBoxBase::GetCountPerPage() const
-{
-    return GetClientSize().y / GetLineHeight();
-}
 #endif // wxUSE_LISTBOX