Bluetooth: Remove unneeded mgmt_connectable function

The mgmt_connectable function has been used to ensure that the right
actions to HCI_CONNECTABLE are taken when the HCI_Write_Scan_Enable
command is triggered by something else than mgmt. The only other user
that we really care about is the HCISETSCAN ioctl code, so we can
actually more simply perform the needed changes there instead.

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