Follow-up to RHDHBUGS-1000.

In https://github.com/backstage/charts/pull/287 , we contributed a change in the upstream Backstage chart to unset the pullPolicy field, so that the default pull behavior could be honored, as depicted in https://kubernetes.io/docs/concepts/containers/images/#imagepullpolicy-defaulting

Now that https://github.com/backstage/charts/pull/287 has been merged (and the change pulled in this Renovate PR), we should do the same in our RHDH chart by unsetting the pullPolicy fields previously set as part of https://github.com/redhat-developer/rhdh-chart/pull/248/files#diff-8060d9a38501197eddb026fced7fe56f104f4f5143210c6d47781b831a7097f4