Revert "mime-types: use g_content_type_is_a for mime support"

This reverts d7e45a26

It gives more problems than it solves. For example docx is a subtype of zip,
which makes autoar report it as supported, when it's not.

So it's better to use the subtypes that autoar supports directly.

https://bugzilla.gnome.org/show_bug.cgi?id=780214