Delete unused secrets in service cluster default namespace

[fejta]

ref #15806

/assign

• ci-service-account, [email protected], 2152c16d13db (super old)
• tide-history-service-account, [email protected] (migrating to workload identity) Make tide use workload identity for GCP auth #16465
• service-account, [email protected] (migrating to workload identity) Prepare grandmatriarch, boskos-janitor to use workload identity #16466 Use workload identity for grandmatriach, boskos-janitor #16475
• triage-service-account Stop requiring triage to have secret credentials #16462

[fejta]

fejta@fejta3:~/src/gh/test-infra$ gsutil ls -l gs://k8s-prow
  12990770  2020-02-25T18:40:54Z  gs://k8s-prow/tide-history.json
      1101  2020-02-25T18:18:09Z  gs://k8s-prow/tide-status-checkpoint.yaml
TOTAL: 2 objects, 12991871 bytes (12.39 MiB)
fejta@fejta3:~/src/gh/test-infra$ kubectl --context=prow delete secrets tide-history-service-account
secret "tide-history-service-account" deleted
fejta@fejta3:~/src/gh/test-infra$ date
Tue 25 Feb 2020 10:42:02 AM PST

[fejta]

fejta@fejta3:~/src/gh/test-infra$ grep -R "secretName: service-account" prow/cluster 
prow/cluster/grandmatriarch.yaml:          secretName: service-account
prow/cluster/boskos-janitor.yaml:          secretName: service-account
prow/cluster/boskos-janitor.yaml:          secretName: service-account
fejta@fejta3:~/src/gh/test-infra$ kubectl --context=prow get deployments -o yaml | grep service-account
fejta@fejta3:~/src/gh/test-infra$ kubectl --context=prow delete secrets service-account && date
secret "service-account" deleted
Tue 25 Feb 2020 10:53:50 AM PST

(grandmatriarch and boskos are in the build cluster)