Remove legacy relation rename, bump Import/Export version

What does this MR do?

Remove redundant legacy relations renames.
It will make our Import/Export faster.

The 'actual' change happens in spec/fixtures/lib/gitlab/import_export/project.json,
everything else is the version number bump + result of the bundle exec rake gitlab:import_export:bump_version as mentioned in https://docs.gitlab.com/ee/development/import_export.html

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content