[gtkmm] ButtonBox: Remove apparently-useless BUTTONBOX_DEFAULT_SPACING.

[Murray Cumming <murrayc src gnome org>]

commit 23cc5e328c91ee2bec70ad3db26050dace462d04
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Apr 27 13:58:09 2017 +0200

    ButtonBox: Remove apparently-useless BUTTONBOX_DEFAULT_SPACING.

 gtk/src/buttonbox.hg |    4 ----
 1 files changed, 0 insertions(+), 4 deletions(-)
---
diff --git a/gtk/src/buttonbox.hg b/gtk/src/buttonbox.hg
index 89e3122..388ce45 100644
--- a/gtk/src/buttonbox.hg
+++ b/gtk/src/buttonbox.hg
@@ -24,10 +24,6 @@ _PINCLUDE(gtkmm/private/box_p.h)
 namespace Gtk
 {
 
-// This is a #define in GTK+, and unrelated to the GtkButtonBoxStyle enum.
-/** @ingroup gtkmmEnums */
-enum { BUTTONBOX_DEFAULT_SPACING = -1 };
-
 
 /** A container for arranging buttons.
  *