Up until v6.0, the chunk splitter is hitting an invariant in case the configured chunk size for a collection is below 1MB. With the changes committed under SERVER-66078, the session collection default chunk size has been decreased to 200KB. Purpose of this ticket is to disable auto-split for the sessions collection in order to avoid hitting the invariant when upgrading to a version with the "no more auto splitter" feature flag active. This is safe because when upgrading, data-size aware balancing will start hence the auto-splitter can be safely disabled.