Remove the issue about optional shadow root. (#392)

Since we serialize shadow root only for elements and never for other node types,
`null` value means here that shadow root of this element is really `null`.

Author: lutien

index.bs

@@ -2203,9 +2203,6 @@ reference to <code>childNodes</code>, or it should be renamed to <code>childrenC
 Issue: <code>nodeValue</code> can be null. Omit the field in such a case, or adjust type to allow
 null value.
 
-Issue: <code>shadowRoot</code> field is optional. Omit it in case of |shadow root| is null, or
-remove optional flag from the field.
-
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