Bluetooth: Clean up structures left unused

With the use of the new structures and lists for the SMP LTK's
we may remove some code that is now unused. No need to have extra
fields of information inside link_key now that it is only used
for Link Keys.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>