Cloud DNS requires every managed zone to have an NS record at all times.
This means if people want to manage their own NS records, we need to add
their new record and remove the old one in the same call. It also means
we can't delete NS records, as we wouldn't know what to replace it with.
So deleting of NS records short-circuits. For the case of `terraform
destroy`, this prevents the error. It does mean if the user explicitly
tries to remove their NS zone from their project, it silently does
nothing, but that's unavoidable unless we want to A) restore a default
value (and it looks like the default values change from zone to zone?
And that is arguably just as unexpected?) or B) let the (arguably more
reasonable) `terraform destroy` case be impossible.
