Stop trying to pull the Protokube image

[hakman]

Protokube is always side-loaded. It is never pushed to a Docker registry, so there is no reason to pull it.

protokubeImage:
  hash: 8f09e5b87f6df65e1579c56775f7c9c6ddac315579f367129c0ed297c46840f5
  name: protokube:1.18.0
  sources:
  - https://artifacts.k8s.io/binaries/kops/1.18.0/images/protokube.tar.gz
  - https://github.com/kubernetes/kops/releases/download/v1.18.0/images-protokube.tar.gz
  - https://kubeupv2.s3.amazonaws.com/kops/1.18.0/images/protokube.tar.gz

[k8s-ci-robot]

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[johngmyers]

/lgtm