remove TOC numbering from PDF export

The default confluencedefaultpdf.css includes TOC numbering, eg

div.toclvl1:before {
    content:  counter(chapter) "." counter(section) " ";
    counter-increment:section;}

However, it is impossible to number sections across conf pages (see CONF-19430).
Therefore the TOC numbering is inconsistent with the rest of the generated PDF and should be removed