Remove dead code in theme_print_pdf_dompdf_footer

This function is no longer called and it was commented out a while ago:

  // It seems dompdf can't handle footers cleanly, so disable the following.
  /*  $html = theme('print_pdf_dompdf_footer', array('html' => $html)); */

It is potentially insecure so it should be removed. This issue was reviewed by the security team and can be public.