Remove unnecessary call to get_thread_db_info

Remove unnecessary call to get_thread_db_info

In thread_db_detach, we call get_thread_db_info to first check if there
exists a thread_db_info entry for the pid to detach.  If there is, then
we call delete_thread_db_info.  It's unnecessary to call
get_thread_db_info in the first place, since delete_thread_db_info
handles the case where no thread_db_info entry exist for the given pid.

gdb/ChangeLog:

	* linux-thread-db.c (thread_db_detach): Remove call to
	delete_thread_db_info.