Remove explicit cookieSecret and proxy.secretToken for staging

[yuvipanda]

With jupyterhub/zero-to-jupyterhub-k8s#1993,
these are no longer required to be explicitly set. We remove
both just for staging hubs - doing so for production
hubs will cause everyone to be logged out (cookieSecret) and
temporarily disconnected (proxy.secretToken) so we aren't
doing that yet

[yuvipanda]

Ok, so to rotate the secrets, you need to:

1. Delete the hub secret in each namespace,
2. Do a deploy.

I've turned off CircleCI for now, so this is manual.

I've deleted the hub secret with:

 k get ns | rg staging | awk '{ print $1 }' | parallel k delete secret hub -n

And am deploying with:

ls deployments | xargs -I{} -L1 hubploy deploy {} hub staging   

[yuvipanda]

I've just paid for CircleCI out of pocket so we can have it again in private.