Bluetooth: Remove unneeded mgmt_write_scan_failed function

The Set Connectable/Discoverable mgmt handlers use a hci_request with a
proper callback to handle the HCI command sending. It makes therefore
little sense to have this extra function to be called from hci_event.c
for command failures.

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