Clean up legacy JSON fixtures and tree_saver for Import/Export

Follow up from !110729 (comment 1314263907)

• Remove the following fixtures for the legacy JSON:
  • spec/fixtures/lib/gitlab/import_export/complex/project.json
  • spec/fixtures/lib/gitlab/import_export/designs/project.json
  • spec/fixtures/lib/gitlab/import_export/group/project.json
  • spec/fixtures/lib/gitlab/import_export/group_exports/complex/group.json
  • spec/fixtures/lib/gitlab/import_export/group_exports/no_children/group.json
  • spec/fixtures/lib/gitlab/import_export/group_exports/visibility_levels/internal/group.json
  • spec/fixtures/lib/gitlab/import_export/group_exports/visibility_levels/private/group.json
  • spec/fixtures/lib/gitlab/import_export/group_exports/visibility_levels/public/group.json
  • spec/fixtures/lib/gitlab/import_export/invalid_json/project.json
  • spec/fixtures/lib/gitlab/import_export/light/project.json
  • spec/fixtures/lib/gitlab/import_export/milestone-iid/project.json
  • spec/fixtures/lib/gitlab/import_export/multi_pipeline_ref_one_external_pr/project.json
  • spec/fixtures/lib/gitlab/import_export/with_invalid_records/project.json
• Remove lib/gitlab/import_export/legacy_relation_tree_saver.rb and the related spec in spec/lib/gitlab/import_export/legacy_relation_tree_saver.rb (we missed this in in !105087 (merged))