Update centos-art.sh script's tuneup module.
    
    - Previously, when cleaning up HTML files, tuneup module used the cleanup-before.sed and
      cleanup-after.sed files for applying sed commands to the file being
      processed. The commands set in those sed files remain from mailman's
      template customization and aren't really needed in most generic HTML
      files the tuneup module exist for. This commit removes both
      cleanup-before.sed and cleanup-after.sed files from tuneup module
      and the line that executed them from xhtml_setCleanUp function.
    
    - Previously, when cleaning up XHTML files, tuneup module generated
      wrong HTML preambles by creating a  line. This
      was because xmllint was processing the file as HTML not XML. This
      commit permits you to cleanup both HTML and XHTML files without such
      problem. To do this, a grep line was added to verify whether the
      -//W3C//DTD XHTML 1.0 Strict//EN definition exists or not in the
      file being processed. Based on it, when XHTML definition doesn't
      exist, xmllint treats the file as XML and, when it doesn't exist,
      xmllint treats the file as HTML.