Some issues with the Hibernate example

I'm currently translating some of the Java/Hibernate examples to jOOQ and found a few issues that you may or may not want to address:

### `CustomerService::deleteCustomer` fails when orders have `OrderProduct` entries:

This query here:

https://github.com/cockroachdb/examples-orms/blob/8aacfca23a8b91667aad846e7e41e4e14c7658cc/java/hibernate/src/main/java/com/cockroachlabs/services/CustomerService.java#L83

Will fail in case there are entries in `order_products`, which does not have `ON DELETE CASCADE` clauses on its foreign keys by default (I think). I'm not sure if this is desired here.

### `OrderService::deleteOrder` fails because of wrong query:

This query here:

https://github.com/cockroachdb/examples-orms/blob/8aacfca23a8b91667aad846e7e41e4e14c7658cc/java/hibernate/src/main/java/com/cockroachlabs/services/OrderService.java#L83

Will fail because the `Order` entity doesn't have an `order_id` column. I think the intention here was to delete the record in `order_products`