Remove notification settings in batches

## What does this MR do?

This improves the performance of the migration `db/migrate/20160603180330_remove_duplicated_notification_settings.rb` by removing duplicate rows in batches instead of using a single big `DELETE FROM` query.

## Why was this MR needed?

The original migration would locally take 45 minutes to complete, possibly up to hours on GitLab.com and similar setups.

## What are the relevant issue numbers?

#18289

See merge request !4529