[OMP] Subjects are incorrectly labelled as BIC in ONIX exports

[kaitlinnewson]

Describe the bug
In the XML exported by the ONIX export plugin, subjects are labelled as BIC subjects (assigned code value "12" in SubjectSchemeIdentifier) - however, this is a free-text field in OMP's publication metadata and subjects could follow any scheme or none (BIC is not implemented or enforced).

Since the subjects may not follow BIC, we should update the exports accordingly by either not exporting subjects at all, or labelling them as keywords.

To Reproduce
Steps to reproduce the behavior:

1. Create a publication with subjects filled in (ensure that subject metadata is enabled in the press)
2. Export ONIX XML using the ONIX export plugin
3. Review the XML, noting the subjects metadata values

What application are you using?
OMP, all versions

Additional information

• Location where subject metadata is created in the ONIX export plugin
• ONIX List 27 (Subject Scheme Identifier)
• Related issue: [OMP] Use of ONIX BIC/Thema subject categories #9906