Use the user's web browser for PDF export by default, instead of relying on Docker

Performing the PDF export using a headless Chrome browser running inside a Docker container is nicer for the user (i.e. consistent results) but requires setting up Docker on the machine running XWiki. Administrators often forget to do this, leading to a failing PDF export. It's better to rely on the user's browser by default and let administrator enable the Docker-based PDF export after they setup Docker properly.