A recent Docker image update broke the test, as xgettext now prints
the following warning:

warning: a fallback ITS rule file '/usr/share/gettext-0.21/its/metainfo.its'
is used; it may not be in sync with the upstream

That is completely unrelated to what the test is meant to catch and
could be fixed by adding appstream to the image, but considering that

 - the test didn't actually catch the last template string regression
 - we no longer allow template strings in files that include translatable
   strings (and enforce that with a CI job)
 - as of gettext 0.20.2, the template handling really really is fixed
   (we'll see)

let's remove the test rather than piling up more stuff in the container
image.

!1399