Remove legacy json formatted Import support from Import/Export codebase

Summary

Project/Group Import/Export introduced .ndjson formatted Import/Export, which splits relations into separate .ndjson files instead of having everything in one project.json file.

3-4 milestones worth of time passed since both legacy and ndjson imports were supported and legacy format can now be cleaned up to fully migrate to ndjson only.

Proposed approach

1. Remove legacy code branching from Group Import/Export lib/gitlab/import_export/group
2. Remove legacy code from Group Import/Export Gitlab::ImportExport::Group::LegacyTreeRestorer, Gitlab::ImportExport::Group::LegacyTreeSaver, etc.
3. Remove legacy branching from Project Import/Export lib/gitlab/import_export/project
4. Remove legacy code from Project Import/Export Gitlab::ImportExport::JSON::LegacyWriter, Gitlab::ImportExport::JSON::LegacyReader
5. Remove old testing fixtures

The above can be split into multiple MRs, 1 MR per 1-2 points.