Delete BBM backfill identifier_external_ids for vulnerability_reads

What does this MR do and why?

Delete the BBM to backfill vulnerability_reads.identifier_external_ids with identifier.external_value data.

We do not require this migration and the removal steps were verified and confirmed with Database team in !160594 (diffs).

This is the Second MR to cleanup involving the step:

• In Release N, add a new post-deployment migration, to delete the existing batched migration. Delete the existing batched background migration using the delete_batched_background_migration method at the start of the #up method to ensure that any existing runs are cleaned up.

1st MR is !162209 (merged)

Related to #460080

Database

> bundle exec rake db:migrate:up:main VERSION=20240808165039

main: == [advisory_lock_connection] object_id: 127720, pg_backend_pid: 16550
main: == 20240808165039 CleanupBackfillIdentifierExternalIdsOfVulnerabilityReads: migrating
main: == 20240808165039 CleanupBackfillIdentifierExternalIdsOfVulnerabilityReads: migrated (0.0477s)

main: == [advisory_lock_connection] object_id: 127720, pg_backend_pid: 16550

> bundle exec rake db:migrate:down:main VERSION=20240808165039

main: == [advisory_lock_connection] object_id: 127720, pg_backend_pid: 17095
main: == 20240808165039 CleanupBackfillIdentifierExternalIdsOfVulnerabilityReads: reverting
main: == 20240808165039 CleanupBackfillIdentifierExternalIdsOfVulnerabilityReads: reverted (0.0032s)

main: == [advisory_lock_connection] object_id: 127720, pg_backend_pid: 17095

MR acceptance checklist

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

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.