base: don't check unicity when migrating Person.reference to user_id

This activity is spawned on all nodes, which cause too many conflicts.
We can take the risk here, assuming that references were already OK
before migration.
2 jobs for <span class="ref-name">fix/setId_serialize</span>
Status Job ID Name Coverage
  External
running ERP5.CodingStyleTest-TestRunner1

5943:24:39

running ERP5.UnitTest-TestRunner1

5943:24:24