Fix 0003504: project cookie not cleared at logout

M core/helper_api.php

• (helper_clear_pref_cookies): Function was clearing cookies using the wrong cookie names.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2544 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9