Remove unused td_thr_validate code

Remove unused td_thr_validate code

linux-thread-db.c initializes td_thr_validate but never uses it.
This commit removes this dead code.

gdb/ChangeLog:

	* linux-thread-db.c (struct thread_db_info)
	<td_thr_validate_p>: Remove field.
	(try_thread_db_load_1): Remove initialization for the above.