drop unused sig-auth staging registry

[upodroid]

Part of #8663

@Ligitt @dims

There are no active github repositories pushing jobs to this staging registry for several years.(last prod promo was in 2022)

https://github.com/search?q=repo%3Akubernetes%2Ftest-infra%20k8s-staging-sig-auth&type=code

I found this on Slack https://kubernetes.slack.com/archives/CHGFYJVAN/p1708975545162689

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[liggitt]

I don't really object to dropping this, but we should clean up references from things like https://github.com/kubernetes/kubernetes/blob/3eeb838aba6d5b311dcfb82f79bd55adfbd45f99/staging/src/k8s.io/pod-security-admission/webhook/manifests/50-deployment.yaml#L31 first

[k8s-ci-robot]

PR needs rebase.

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