window: don't change bottom margin for automatic CSD

We'll use a style class to be able to give this a different appearance,
but for the time being we don't really need to give this such different
margin.

https://bugzilla.gnome.org/show_bug.cgi?id=706592

gtk/gtkwindow.c

@@ -5166,7 +5166,6 @@ update_window_buttons (GtkWindow *window)
 
           box = gtk_box_new (GTK_ORIENTATION_HORIZONTAL, 0);
           gtk_widget_show (box);
-          gtk_widget_set_margin_bottom (box, 6);
           if ((direction == GTK_TEXT_DIR_LTR && i == 0) ||
               (direction == GTK_TEXT_DIR_RTL && i == 1))
             gtk_style_context_add_class (gtk_widget_get_style_context (box), "left");