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/gtkbbox.h

@@ -42,10 +42,8 @@ G_BEGIN_DECLS
 #define GTK_IS_BUTTON_BOX(obj)          (G_TYPE_CHECK_INSTANCE_TYPE ((obj), GTK_TYPE_BUTTON_BOX))
 #define GTK_IS_BUTTON_BOX_CLASS(klass)  (G_TYPE_CHECK_CLASS_TYPE ((klass), GTK_TYPE_BUTTON_BOX))
 #define GTK_BUTTON_BOX_GET_CLASS(obj)   (G_TYPE_INSTANCE_GET_CLASS ((obj), GTK_TYPE_BUTTON_BOX, GtkButtonBoxClass))
-  
 
-#define GTK_BUTTONBOX_DEFAULT -1
- 
+
 typedef struct _GtkButtonBox       GtkButtonBox;
 typedef struct _GtkButtonBoxPriv   GtkButtonBoxPriv;
 typedef struct _GtkButtonBoxClass  GtkButtonBoxClass;