Fix opening of AppImage files
This hack in the mimetype detection logic in Ark was added to work around an old bug in shared-mime-info about ISO files detected as text files.
It should not be needed anymore nowadays, since even Debian 11 ships shared-mime-info 2.0 which should already contain the fix.
This check currently even breaks the detection of application/x-iso9660-appimage (since it inherits the ISO mimetype), so it's about time we just drop it.