Forcibly exclude fedora-jam-kde-theme from KDE images
    
    2.0.0 went stable in F31, but it's kinda messed up, it provides
    kde-settings, and when building this image, anaconda actually
    prefers it over the 'real' kde-settings, which breaks the desktop
    background among other things. 3.0.0 should fix it but it's still
    in updates-testing, so let's forcibly exclude it for now.