We need this because in DefaultTaxonomy (Taxonomies bundle) class we have:
```php
$this->setRoot(new DefaultTaxon());
```
and after remove Taxonomy mapping from core bundle i have error when creating new taxonomy:
```
[Doctrine\ORM\ORMException]                                                                                                                  
  Found entity of type Sylius\Bundle\TaxonomiesBundle\Entity\DefaultTaxon on association Sylius\Bundle\TaxonomiesBundle\Entity\DefaultTaxonom  
  y#root, but expecting Sylius\Bundle\CoreBundle\Entity\Taxon
```