Bluetooth: Don't store debug keys if flag for them is not set

Instead of waiting for a disconnection to occur to remove a debug key
simply never store it in the list to begin with. This means we can also
remove the debug keys check when looking up keys in
hci_link_key_request_evt().

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