Feature flag cleanup - `bulk_imports_batched_import_export`

What does this MR do and why?

The bulk_imports_batched_import_export feature flag was introduced in 16.2 and is now being removed after serving its purpose.

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

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 #406559 (closed)