Get rid of obsolete dokuwiki plugins

We have some old unmaintained dokuwiki plugins, triggering warnings in logs. We may get rid of them.

Some old pages/dokuwiki old revisions can be broken after removing theses plugins, but I'm ok with that.

• https://www.dokuwiki.org/plugin:cellbg : Not used, to remove: Disabled ✅
• https://www.dokuwiki.org/plugin:definitionlist: Not used, to remove: Disabled ✅
• https://www.dokuwiki.org/plugin:dl: Used 😞 It is used on https://docs.xfce.org/xfce/getting-started for example (Usage of ? / !. Maybe we can use standard "lists" ?
• https://gitlab.xfce.org/www/wiki.xfce.org/-/blob/master/lib/plugins/xfcesubs/: To catch XfceBug and XfceSvnRev patterns. To remove! Disabled ✅
• https://gitlab.xfce.org/www/wiki.xfce.org/-/blob/master/lib/plugins/xfcedocs/: Not sure to understand what it do. Seems to strong the text if started with {gui or {key . It seems that it catch the <figure patterns too ?

I don't understand xfcedocs purpose. But it is still used on recent pages, like https://docs.xfce.org/xfce/getting-started (search for <figure)