Only use LIBXML_HTML_NODEFDTD is available (#4486)
Only use LIBXML_HTML_NODEFDTD is available (#4486)

* Only use LIBXML_HTML_NODEFDTD is available

* Actually remove constants

Co-Authored-By: Pierre Gordon <91995e9b2246270f4ecfe80d34169c75f319bb5e@users.noreply.github.com>

Co-authored-by: Pierre Gordon <91995e9b2246270f4ecfe80d34169c75f319bb5e@users.noreply.github.com>
ampproject/amp-toolbox-php,ampproject/amp-toolbox-php