Remove bulk_queue_bg_migration_jobs_by_range

What does this MR do and why?

Delete the bulk_queue_background_migration_jobs_by_range helper method. This method has gone unused for some time, and most likely we wouldn't want to schedule a background migration without a delay between jobs.

This came out of the work to make background migrations work with multiple databases in &6840 (closed). It seems not worth it to update this method to work with multiple databases, especially after cleaning up old migrations in #331248 (closed).

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