tree-store: Simplify gtk_tree_store_clear_traverse

GtkTreeStore's clear method uses a recursive helper, gtk_tree_store_clear_traverse(), to do the actual clearing. A comment states that it was ripped off from g_node_traverse_post_order(), but it kept some unnecessary logic from that source. In particular, it returned a gboolean value, which was always (although not immediately obviously) FALSE.

This commit removes the return value and eliminates an unnecessary branch in the code.