main: Remove mention of chapters plugin

The "autoload chapters" setting doesn't do anything since the chapters
plugin was first disabled with the UI redesign in GNOME 3.12, and then
removed.
4 jobs for master in 15 minutes and 45 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #228518
build-fedora

00:04:17

passed #228519
flatpak

00:14:56

 
  Review
passed #228520
review

00:00:48

manual #228521
allowed to fail manual
stop_review