Finalize and remove BackfillDuoCoreForExistingSubscription background migration

Problem to solve

The BackfillDuoCoreForExistingSubscription background migration is no longer needed as mentioned in issue #539075. The test for this migration is currently quarantined due to flakiness.

Proposal

1. Finalize the batched background migration following the documentation
2. Delete the background migration code according to the guidelines

Tasks

• Finalize the background migration
• Delete the background migration code, including spec files
• Update any related documentation