Drop feature flag: import_bulk_inserts

What does this MR do?

Closes #215147 (closed)

Removes feature flag import_bulk_inserts

Post-deploy run:

/chatops run feature delete import_bulk_inserts --staging
/chatops run feature delete import_bulk_inserts

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content