Cleanup Background Migration for Orphaned Deployments

Problem

Previously, we shipped the background migration to delete orphaned deployments. This migration can only be cleaned up in the next milestone, which is %14.2. We should clean up to complete the task.

Proposal

• Clean up the background migration.
• Update FK constraint to validate all rows https://docs.gitlab.com/ee/development/database/add_foreign_key_to_existing_column.html#prevent-invalid-records