[migration] Avoid unnecessary intermediary commit when migrating a schema
Those are low-hanging fruit following changes in ac74476d686c (Fix addition of
entity type including boundary constraints on its own attributes): this is no
more necesary to commit the get a new relation type definition in the schema.
At some point we could/should probably do more on this topic to avoid
intermediary commit on e.g. entity type addition, but this requires more work.