Remove `project_export_as_ndjson` and `project_import_ndjson` feature flags

The feature flags project_export_as_ndjson and project_import_ndjson have been introduced in %12.10 and are enabled by default since ~2 years. Let's remove the code reference and check if there is related legacy code that can be removed along (such as Gitlab::ImportExport::Json::LegacyWriter)

For reference, we have recently removed the feature flags group_export_ndjson and group_import_ndjson in #383682 (closed)

TODOs

• Remove references to project_export_as_ndjson and project_import_ndjson feature flags
• Remove the LegacyWriter and LegacyReader
• Update documentation that import/export, starting in 15.10, will no longer support old .json exports for projects (We'd have to double check which is the last version supported)