Remove storage class from logging tutorial

Specifying the storage class in the logging tutorial triggers a failure on platforms where it does not exist. We should remove it so it just uses the default.

https://docs-staging.couchbase.com/operator/current/tutorial-couchbase-log-forwarding.html#configure-the-couchbase-cluster

  volumeClaimTemplates:

  - metadata:

      name: couchbase

    spec:

      storageClassName: standard

      resources:

        requests:

          storage: 1Gi

Remove that `storageClassName`. Add a call out to storage class docs as well.