nptl_db: Remove stale `match_pid' parameter from `iterate_thread_list'

Complement commit c579f48e ("Remove cached PID/TID in clone") and
remove the `match_pid' parameter not used by `iterate_thread_list' any
longer.  Update call sites accordingly.

	* nptl_db/td_ta_thr_iter.c (iterate_thread_list): Remove
	`match_pid' parameter.
	(td_ta_thr_iter): Update accordingly.