Reintroduce assignment schema change without dropping the old column

Migrations should keep the old columns around as during upgrades there might be older versions still using them. This commit removes the migration that drops the old assigned column to prevent breakage during upgrades. The migration can be reintroduced later after the Praefects have been upgraded to use the new repository_assignments table. This would be 13.8 release with our current policy on zero downtime upgrades.

Original MR: !2848 (merged)