Drop explicit UNIQUE_RELATIONS list from Import

What does this MR do?

The idea is to drop an explicit UNIQUE_RELATIONS list from the codebase in favor of calculating it in place.

Details in #36647 (closed)

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

N/A

Security

N/A