Bluetooth: Remove unused hci_find_ltk function

Now that LTKs are always looked up based on bdaddr (with EDiv/Rand
checks done after a successful lookup) the hci_find_ltk function is not
needed anymore. This patch removes the function.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>