tests: Remove qcow2 encryption from storagevol tests
    
    We're about to disallow creation of a qcow2 encrypted storage
    volume, so let's remove the qcow encryption element from the
    tests which are testing whether other format='qcow2' related
    features work properly.
    
    Signed-off-by: John Ferlan <jferlan@redhat.com>
    ACKed-by: Michal Privoznik <mprivozn@redhat.com>