Bluetooth: Remove no longer needed mgmt_new_settings() function

The mgmt_new_settings() function was only needed to handle the
error case when re-enabling advertising failed. Since that is
now handled internally inside the management core, this function
is not needed anymore. So just remove it.

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