trivial: Use NoDisplay instead of Hidden for importer .desktop file

The Hidden field basically voids the .desktop file and is intended to be
used by humans who want to disable an application completely (including
its mime associations).
So, setting this to Hidden was actively harmful before. Projects should
use NoDisplay to set defaults.

Also removed the comment field added in the last commit, because now
that the file is hidden, AppStream will not pick it up. This prevents
translators doing unnecessary localization.