Remove work_items_bulk_edit feature flag

What does this MR do and why?

Removes the work_items_bulk_edit wip feature flag and defaults the work item bulk edit experience to use the newer GraphQL based experience.

References

Screenshots or screen recordings

No visual changes

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Relates to #551187 (closed)