As of Nomad 1.6.0, Nomad client agents send their secret with all the
RPCs (other than registration). But for backwards compatibility we had to keep
a legacy auth method that didn't require the node secret. We've previously
announced that this legacy auth method would be removed and that nodes older
than 1.6.0 would not be supported with Nomad 1.9.0.
This changeset removes the legacy auth method.
Ref: https://developer.hashicorp.com/nomad/docs/release-notes/nomad/upcoming#nomad-1-9-0
