[cssom] Remove comment that now is invalid per CSSWG resolution.

getComputedStyle returns styles in the node's document per the resolution in
w3c/csswg-drafts#1548.

css/cssom/getComputedStyle-dynamic-subdoc.html

@@ -5,10 +5,6 @@
 <link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
 <script src=/resources/testharness.js></script>
 <script src=/resources/testharnessreport.js></script>
-<!--
-  NOTE: The way this test is written doesn't match the spec, but matches all
-  implementations, see https://github.com/w3c/csswg-drafts/issues/2403
--->
 <iframe id="frm" style="width: 100px; height: 100px"></iframe>
 <script>
 test(function() {