Remove staging zaaksysteem from ZaakBrug

[philipsens]

          > Can I update the staging openzaak in the helm chart, if this works, @MLenterman?

It turned out that staging OZ was not a viable solution for the ZGW to ZDS translation, so it is no longer being used. I think we can remove all the staging related stuff from the chart.

Originally posted by @MLenterman in wearefrank/zaakbrug#263 (comment)