Change session logout to kill only the cookie
    
    Removing the ccache goes too far as it will cause unrelated sessions to
    fail as well, this is a problem for accounts used to do unattended
    operations and that may operate in parallel.
    
    Fixes https://fedorahosted.org/freeipa/ticket/6682
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>