comics: Fix extra ";" leading to a warning during installation

The concatenated mime-types end up with a ...mime-type;;mime-type...
pattern, an empty mime-type, which update-desktop-database doesn't
like.

Error in file "/usr/share/applications/evince.desktop": "" is an invalid MIME type ("" does not contain a subtype)

See https://bugzilla.redhat.com/show_bug.cgi?id=1471474

https://bugzilla.gnome.org/show_bug.cgi?id=785026