fix: remove default BookKeeper stats provider

[mattisonchao]

Motivation

• Pulsar images that include [fix][sec][branch-4.0] Upgrade to Jetty 12.1.8 to address several CVEs apache/pulsar#25534 no longer include org.apache.bookkeeper.stats.prometheus.PrometheusMetricsProvider.
• The charts currently render that BookKeeper stats provider by default, which can cause BookKeeper pods to fail with ClassNotFoundException when using newer Pulsar images.
• The image already carries the appropriate default BookKeeper configuration, so the chart should avoid forcing a provider class.

Modifications

• Removed the default statsProviderClass from the BookKeeper common config template.
• Removed the default statsProviderClass from the BookKeeperCluster template in both sn-platform and sn-platform-slim.

Testing

• Rendered sn-platform-slim with helm template and confirmed no statsProviderClass or BookKeeper Prometheus provider class is emitted by the rendered BookKeeperCluster.

[github-actions]

@mattisonchao:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)