ModelSQL.delete call always Translation.delete_ids
When deleting a records, trytond tries to delete the linked translations even if the model does not have any translatable fields. The method has a cost as it always makes search queries.
So it will be better to not make this call if there are no translatable fields.
I do not think there is any issue regarding if the model had previously (in older version) translatable fields because it should be the responsibility of the migration.