Find and Remove in admincp/index.php:
<script type="text/javascript">
<!--
if (typeof(vb_version) != "undefined" && isNewerVersion("<?php echo $vbulletin->options['templateversion']; ?>", vb_version))
{
var current_version = "<?php echo $vbulletin->options['templateversion']; ?>";
var latest_string = "<?php echo $vbphrase['latest_version_available_x']; ?>";
var current_string = "<?php echo $vbphrase['you_are_running_vbulletin_version_x']; ?>";
var download_string = "<?php echo $vbphrase['download_vbulletin_x_from_members_area']; ?>";
document.writeln('<table cellpadding="4" cellspacing="0" border="0" align="center" width="90%" class="tborder"><tr><td class="tcat" align="center"><b><?php echo $vbphrase['there_is_a_newer_vbulletin_version']; ?></b></td></tr><tr><td class="alt1" align="center">');
document.writeln('<p><b><a href="http://www.vbulletin.com/forum/showthread.ph' + 'p?p=' + vb_announcementid + '" target="_blank">' + construct_phrase(latest_string, vb_version) + "</a></b></p>");
document.writeln('<p>' + construct_phrase(current_string, current_version.bold()) + '</p>');
document.writeln('<p><a href="http://members.vbulletin.com/" target="_blank">' + construct_phrase(download_string, vb_version.bold()) + '</a></p>');
document.writeln('</td></tr></table>');
}
//-->
</script>