migrations: remove unused campaigns tables
Created by: LawnGnome
In #13001, we migrated campaigns from our old, pre-spec workflow to the new, spec-based workflow. As an escape hatch for support, we preserved the old tables in case the migration failed and we needed to retrieve unmigrated data.
Several versions later, we believe all campaign users have upgraded, and no issues have been reported with that migration. Therefore, it's time to burn the shipyards that built the boats we burned in #13001.
Closes #16223.