Clean up background_migration/backfill_namespace_id_of_vulnerability_reads

This migration was removed in the last mass migration clean up done by the database team.

As such the files associated with the migration are redundant and we can just delete them:

• lib/gitlab/background_migration/backfill_namespace_id_of_vulnerability_reads.rb
• spec/lib/gitlab/background_migration/backfill_namespace_id_of_vulnerability_reads_spec.rb