tests: Drop a content rating test due to it being in appstream-glib

See the previous commit, and
https://github.com/hughsie/appstream-glib/pull/364.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
5 jobs for content-rating-rationalisation in 24 minutes and 48 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #727901
debian-stable-x86_64

00:20:56

passed #727911
fedora-x86_64

00:03:52

failed #727847
debian-stable-x86_64

01:00:01

failed #727902
fedora-x86_64

00:05:32

failed #727846
fedora-x86_64

00:03:32