Remove Security Guide EPUB

The EPUB is not getting updated and thus outdated. It's also not easy to
install on mobile devices. Thus remove it.

Change-Id: Ia60754c6e03434d84d27c6e33752cecf28872c76

www/sec/index.html

@@ -132,8 +132,6 @@ function recordOutboundLink(link, category, action) {
 		<div class="span-12">
             <div class="photo"><img width="100%" src="../common/images/openstack-security-guide.jpg"></div>
 			<p><a href="http://docs.openstack.org/security-guide/content/" class="button" onClick="recordOutboundLink(this, 'Outbound Links', 'security-guide.html');return false;" >HTML</a>
-            &nbsp;&nbsp;&nbsp;&nbsp;
-			<a href="openstack-security-guide.epub" class="button" onClick="recordOutboundLink(this, 'Outbound Links', 'openstack-security-guide.epub');return false;" >EPUB</a>
             &nbsp;&nbsp;&nbsp;&nbsp;
             <a href="http://docs.openstack.org/security-guide/security-guide.pdf" class="button" onClick="recordOutboundLink(this, 'Outbound Links', security-guide.pdf');return false;" >PDF</a>
 			</p>