Disable sorting by "relation" column

Overview:

Disable sorting by the "relation" column because this cannot work as there is no "relation" property.

Steps to Reproduce:

1. Click on the "relation" column head
2. This enables sorting by "relation" or toggles its ascending/descending sort order.

However, the "relation" entries don't get sorted.

Neither mod-inventory nor mod-inventory-storage have a "relation" property in the instance record. Sorting by a non-existing property requires a full table scan resulting in bad performance but does not result in the expected order. Example query:

https://folio-goldenrod-okapi.dev.folio.org/inventory/instances?limit=100&query=(keyword all "a*") sortby relation/sort.descending

The front-end should disable the sort option for the "relation" column.