Remove unused libthread_db td_thr_validate reference

Remove unused libthread_db td_thr_validate reference

Native GDB looks up the function td_thr_validate from libthread_db.so
on Linux, but the value is never used.  This commit removes this dead
code.

gdb/ChangeLog:

	* nat/gdb_thread_db.h (td_thr_validate_ftype): Remove typedef.
	* linux-thread-db.c (struct thread_db_info) <td_thr_validate_p>:
	Remove field.
	(try_thread_db_load_1): Remove td_thr_validate initialization.