Unset namespace on NGINX IngressClass (cluster-scoped)

What does this MR do?

Unset namespace on IngressClass (cluster-scoped)

IngressClasses are cluster-scoped: https://kubernetes.io/docs/concepts/services-networking/ingress/#ingressclass-scope

As such, we don't need to set the namespace on them.

For reference, no namespace is set on the object in the upstream chart: https://github.com/kubernetes/ingress-nginx/blob/controller-v1.0.4/charts/ingress-nginx/templates/controller-ingressclass.yaml

Checklist

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