Bluetooth: Add support for removing LTK's when pairing is removed

Instead of having a separated command for removing SMP keys, we use the
Remove Keys command to remove *all* 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>