Remove some leftovers of deprecated functionality

There were some vestiges of the gtk_{h,v}button_box_set_default_layout()
functionality left. These are gone now. I have also removed
the GTK_BUTTONBOX_DEFAULT value in GtkButtonBoxStyle, but the other
values have been kept at their numeric values, to avoid more serious
ABI change.

gtk/gtkvbbox.c

@@ -55,8 +55,6 @@
  * gtk_button_box_set_layout().
  */
 
-static GtkButtonBoxStyle default_layout_style = GTK_BUTTONBOX_EDGE;
-
 G_DEFINE_TYPE (GtkVButtonBox, gtk_vbutton_box, GTK_TYPE_BUTTON_BOX)
 
 static void
@@ -83,10 +81,3 @@ gtk_vbutton_box_new (void)
 {
   return g_object_new (GTK_TYPE_VBUTTON_BOX, NULL);
 }
-
-
-GtkButtonBoxStyle
-_gtk_vbutton_box_get_layout_default (void)
-{
-  return default_layout_style;
-}