Remove references to registry-secret

We used to rely on a registry-secret producdes by sylva units to retrieve kube-job image.

However, this secret is only produced in management-cluster, leading to misleading Warning messages in Workload cluster:

Warning  FailedToRetrieveImagePullSecret  3m53s (x24 over 9m4s)  kubelet            Unable to retrieve some image pull secrets (registry-secret); attempting to pull the image may not succeed.

We should stop generating this secret and using it in kube-(cron)jobs