Bluetooth: Fix hci_remove_ltk failure when no match is found

There is code (in mgmt.c) that depends on the hci_remove_ltk function to
fail if no match is found. This patch adds tracking of removed LTKs
(there can be up to two: one for master and another for slave) in the
hci_remove_ltk function and returns -ENOENT of no matches were found.

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