CLEANUP: connection: drop an uneeded leftover cast

In conn_delete_from_tree() there remains a cast of the toremove_list
to struct list while the introduction of the union precisely was to
avoid this cast. It's a leftover from the first version of patch
5afcb686b ("MAJOR: connection: purge idle conn by last usage") merged
into in 2.9-dev4, let's fix that.

No backport is needed.

src/connection.c

@@ -78,7 +78,7 @@ struct conn_tlv_list *conn_get_tlv(struct connection *conn, int type)
  */
 void conn_delete_from_tree(struct connection *conn)
 {
-	LIST_DEL_INIT((struct list *)&conn->toremove_list);
+	LIST_DEL_INIT(&conn->idle_list);
 	eb64_delete(&conn->hash_node->node);
 }