[css-position-3] Delete redundant heading

Author: fantasai

css-position-3/Overview.bs

@@ -1562,9 +1562,6 @@ Acknowledgments</h2>
 
 <h2 class="no-num" id="changes">Changes</h2>
 
-<h3 id="changes-201502">
-Changes since the 3 February 2015 WD</h3>
-
 	The following significant changes were made since the
 	<a href="https://www.w3.org/TR/2016/WD-css-position-3-20160517/">17 May 2016 Working Draft</a>