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 from v6.0 to v6.1. This is safe because when upgrading to v6.1, data-size aware balancing will start hence the auto-splitter can be safely disabled.