config: drop slaves-order config option

This option was only introduced only to allow keeping the old behavior in RHEL7, while the default order was changed from 'ifindex' to 'name' in RHEL8. The usefulness of this option is questionable, as 'name' together with predictable interface names should give predictable order. When not using predictable interface names, the name is unpredictable but so is the ifindex.