Remove leftover warning

The relevant text was removed in
674b0b2.

spec/custom/index.html

@@ -362,8 +362,6 @@ <h3>Enqueuing and Invoking Callbacks</h3>
     <li>When transitioning back from the user agent code to script, pop the <a>element queue</a> from the <a>processing stack</a> and <a>invoke custom element actions</a> in that queue.</li>
 </ul>
 
-<p class="warning">This needs to become more precise. Find all platform APIs that can create elements? Or invent a new algorithm for creating elements and make sure everyone calls it?</p>
-
 <div class="note">As described, these actions wrap every user agent-implemented method or property accessor. The intended effect is that any lifecycle callbacks, enqueued as a result of running these methods or accessors are invoked prior to returning control back to script. If a method or accessor is known to never enqueue a custom element action, the user agent could choose not to wrap it as a performance optimization.</div>
 
 <p><a data-lt="custom element">Custom elements</a> have an associated <dfn id="dfn-element-is-being-created-flag">element is being created</dfn> flag, initially false.</p>