remove the second ref command on the child_model, we only have to ref it

remove the second ref command on the child_model, we only have to ref it

Sat Nov  2 23:48:52 2002  Kristian Rietveld  <kris@gtk.org>

        * gtk/gtktreemodelsort.c (gtk_tree_model_sort_set_model): remove
        the second ref command on the child_model, we only have to ref it
        once here. (#96851, reported by Jorn Baayen).