Remove `project_export_as_ndjson` and `project_import_ndjson` FFs

What does this MR do and why?

Remove the feature flags project_export_as_ndjson and project_import_ndjson that have been introduced in %12.10 and are enabled by default since ~2 years. Also, remove legacy code such as Gitlab::ImportExport::Json::LegacyWriter and GitLab::ImportExport::Json::LegacyReader.

Screenshots or screen recordings

N/A

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #389888 (closed)