Remove registering of DOMDocument

As stated in documentation of DOMDocument, registering DOMDocument is unnecessary.

Source: https://secure.php.net/manual/en/domdocument.registernodeclass.php#example-6650

src/MicrodataParser.php

@@ -14,7 +14,6 @@ class MicrodataParser
      */
     public function __construct(MicrodataDOMDocument $dom)
     {
-        $dom->registerNodeClass(\DOMDocument::class, MicrodataDOMDocument::class);
         $dom->registerNodeClass(\DOMElement::class, MicrodataDOMElement::class);
 
         $this->dom = $dom;