[ci] Remove unnecessary GSA activation for GAR policy step (#14308)

Since recently adding metadata server support for batch jobs in GCP,
`gcloud` should now "Just Work" using the CI service account in CI jobs
without explicitly configuring it with a key file, so we no longer need
this line. I tested that this succeeds with a dev deploy.

build.yaml

@@ -3932,7 +3932,6 @@ steps:
     scopes:
       - deploy
     script: |
-      gcloud auth activate-service-account --key-file=/gsa-key/key.json
       gcloud artifacts repositories set-cleanup-policies hail \
         --project={{ global.gcp_project }} \
         --location=us \