cleanup legacy GCP janitor jobs

[BenTheElder]

Now that we don't have any unmigrated jobs using GCP projects.

Part of #33226, #33129

[ameukam]

/lgtm
/approve
/hold

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ameukam, BenTheElder

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[BenTheElder]

/hold cancel

[k8s-ci-robot]

@BenTheElder: Updated the job-config configmap in namespace default at cluster test-infra-trusted using the following files:

• key sig-scalability-cleanup.yaml using file config/jobs/kubernetes/sig-scalability/sig-scalability-cleanup.yaml
• key janitors.yaml using file config/jobs/kubernetes/test-infra/janitors.yaml

In response to this:

Now that we don't have any unmigrated jobs using GCP projects.

Part of #33226, #33129

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