Remove staging access to prod images

[yuvipanda]

Currently, the staging compute service account has access to the images we have built in prod. This is the official way to give access to one project's containers for another project's kubernetes clusters. This lets us run tools built with chartpress in staging & prod off the same image.

Ideally, I'd like the projects to be fully self-contained. Once jupyterhub/chartpress#25 is fixed, we can use chartpress to build and push to staging and prod. After that, we can remove access for staging's compute service account from prod.