Plug a placeholder leak in the box adaptor

gtk_widget_destroy() is not the right counterpart for
g_object_ref(), and it is useless anyway to do a ref-unref
pair around gtk_container_remove(), so just drop this.

This fix makes the placeholder instance count go down again
when shrinking the size of a box.

https://bugzilla.gnome.org/show_bug.cgi?id=739751