Drop unneeded pth-devel dependencies
    
    It's not checked on build-time and it seems only needed for macro
    so target application must depend on pth-devel. And what's more
    important, we need to support npth-devel, so we shouldn't require
    both.
    
    Which gets us back to the point that it is just wrong requiring
    (n)pth-devel from libassuan.
    
    Signed-off-by: Igor Gnatenko <ignatenkobrain@fedoraproject.org>