[css-values-4] Remove outdated link to calc-serialization issue

See w3c#6506 (comment)

Author: AtkinsSJ

css-values-4/Overview.bs

@@ -5066,8 +5066,6 @@ Range Checking</h3>
 <h3 id='calc-serialize'>
 Serialization</h3>
 
-	Issue: This section is still <a href="https://lists.w3.org/Archives/Member/w3c-css-wg/2016AprJun/0239.html">under discussion</a>.
-
 	<div algorithm>
 		To <dfn export>serialize a math function</dfn> |fn|: