Remove redundant is_object check

The argument can only be `DOMElement`.

Discovered by bleeding edge PHPStan.

src/Readability.php

@@ -572,14 +572,12 @@ class Readability implements LoggerAwareInterface
      */
     public function cleanStyles(\DOMElement $e): void
     {
-        if (\is_object($e)) {
         $elems = $e->getElementsByTagName('*');
 
         foreach ($elems as $elem) {
             $elem->removeAttribute('style');
         }
     }
-    }
 
     /**
      * Get comma number for a given text.