Consider which PostgreSQL instance to use

During testing in https://gitlab.com/gitlab-org/gl-openshift/gitlab-operator/-/merge_requests/93, I saw errors in the Migrations job related to permissions. I expect this is because we're deploying the Operator's instance of PostgreSQL, but not telling the Helm Chart that we're using 'external' PostgreSQL.

We should decide whether we'll keep the Operator's PostgreSQL instance, or deprecate it and use the instance from the Helm Chart.