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 (closed)

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

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