Fix EZP-21804: Duplicate key in ezcontentobject_attribute when creating new draft with untranslatable field

This PR resolves issue https://jira.ez.no/browse/EZP-21804

When creating Content draft, untranslatable fields were copied from main language.
This was not necessary as in Legacy Storage complete copy is stored for each language. Because ids are respected for field in a language through versions, this resulted in DB constraints violation.

This fixes the issue by copying each field from its own language and also simplifies the implementation a bit.