Remove bogus assert

 - unneeded
 - typechecker and tests happy
 - needed for 3.12 as assert.active default has changed

Author: fredemmott

src/html/XHPHelpers.php

@@ -164,7 +164,6 @@ final public function transferAttributesToRenderedRoot(
         );
       }
     }
-    assert($root instanceof HasXHPHelpers);
 
     $attributes = $this->getAttributes();