Bluetooth: Remove unneeded flag

Remove HCI_LINK_KEYS flag since using HCI_MGMT is enough for test that
user space expects the kernel managing link keys.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Acked-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>