Eliminate find_target_beneath

Eliminate find_target_beneath

Call target_ops::beneath() throughout instead.

gdb/ChangeLog:
2018-06-07  Pedro Alves  <palves@redhat.com>

	* target.h (find_target_beneath): Delete declaration.
	* target.c (find_target_beneath): Delete definition.
	* aix-thread.c: All callers of find_target_beneath adjusted to
	call target_ops::beneath instead.
	* bsd-uthread.c: Likewise.
	* linux-thread-db.c: Likewise.
	* ravenscar-thread.c: Likewise.
	* sol-thread.c: Likewise.
	* spu-multiarch.c: Likewise.