Resolve "Follow-up from "De-dup project tree entries": Remove feature flag"

What does this MR do?

Drops the dedup_project_import_metadata feature toggle.

Does this MR meet the acceptance criteria?

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

This is already rolled out to everyone so no changes here.

Closes #196832 (closed)