Bug 24780: Make items.stocknumber show up in batch item modification

Bug 24780: Make items.stocknumber show up in batch item modification

It looks like the field was intentionally removed from the list
of batch editable fields in the past. This makes sense as we
used to have a unique index on it at some point - but we do have
no more.

This removes the exception so that the invendory number behaves
like the other fields on the batch item edit form.

To test:
- Create some items with and without stocknumber
- Go to tools > batch item modification
- Enter the barcodes of your selected items in the list or
  upload a file with them
- Verify that the stocknumber/inventory number is not showing
  in the item edit form below
- Apply patch
- Reload the page - inventory number is there now
- Batch edit the inventory number and verify it works as expected

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>