Re-evaluate whether we should still automatically apply shipment transitions

Problem/Motivation

Currently, we have an order subscriber automatically applying shipment transitions, and we have this comment:

      // @todo Remove onValidate/onFulfill once there is a Shipments admin UI.
      'commerce_order.validate.post_transition' => ['onValidate'],
      'commerce_order.fulfill.post_transition' => ['onFulfill'],

It might make sense to remove this logic, especially if we implement the change from #3179317: Disable validation for missing 'finalize' and 'cancel' transitions.