Cleanup logic to populate variables no longer needed

[consideRatio]

We had logic to pre-populate variables values, but I've conluded it doesn't seem to be needed any more. Whenever I deploy entirely fresh dashboards and visit them, the variables are up to date. I think they are both refreshed on load and on time range change now.

Related config

query.refresh.onLoad and query.refresh.onTime could be declared explicitly four our variables declared here, but the default seems to be to have those refresh, which is fine and makes us not need the code removed in this PR.

Related issues

• Fixes 404 when trying to access proxy_url in get_label_values #98 as its removing code triggering error
• Fixes Cannot install JupyterHub dashboards #32 as its removing code triggering error

[consideRatio]

I've rechecked all dashboards - this work the same with and without this functionality.

I also think now that this would close #32 and #98, which have errors related to the removed code.

[consideRatio]

I did a third self-review and went for a self-merge!

I worry that the bugs in this code this could cause someone looking to trial these dashboards to fail. I just suggested in https://discourse.jupyter.org/t/grafana-dashboard/25249/3 that a user could try deploy at least one of the dashboards in a non-k8s context - and I guess without knowing that this code could be erroring for a grafana deployed outside k8s context.