gdesktopappinfo: Do not search Comment field

The Comment field provides a user-visible description of the app, which usually contains generic words ("and", "or", "not", "is", ...) that add noise when used for search.

It made some sense to match against the field as a fallback for Keywords, before that key was well established. However that key has been around for years now, so hopefully every app where additional terms are helpful uses it by now.

With that, the downside of added noise outweighs the benefit, so it's time to stop matching on comments.