Stop supporting MARC 596 stub on-order items

[cbeer]

We're currently handling on-order records three ways:

• there's a FOLIO item with the status on-order (and, presumably, a linked order/pieces entry?). This is how orders are natively created in FOLIO (for non-e-resources at least): https://github.com/sul-dlss/searchworks_traject_indexer/blob/6a49ef9/lib/folio_record.rb#L147-L159
• there's a FOLIO holding, and an expected order/pieces entry. This is the way most Symphony orders were migrated to FOLIO: https://github.com/sul-dlss/searchworks_traject_indexer/blob/6a49ef9/lib/folio_record.rb#L183-L195
• there's just a MARC 596 entry, with a code that maps to a library. This is used where there are no items or linked orders. These are probably records that failed to migrate perfectly to FOLIO: https://github.com/sul-dlss/searchworks_traject_indexer/blob/6a49ef9/lib/folio_record.rb#L198-214

We'd like to stop using the MARC 596 lookup, but we suspect there are still some on-order records without the necessary FOLIO data, e.g.: https://searchworks.stanford.edu/view/11746128

In order to continue to show these on-order items in Searchworks, we mightneed to link the order to a library through the locationId (and/or potentially other fields) instead of just the holdingId

[cbeer]

Kevin did some analysis and came up with ~150 instances that won't be indexed. Gryphon-search was ok remediating those objects in the future.

Fixed by #1478.