mkdumprd: Do not add "nfs" dracut module explicitly
    
    Kdump explicitly adds "nfs" dracut module in case of nfs dumping,
    actually in case of nfs dump, nfs is a mount target, and will be
    added into host_fs_types[], thus dracut will add it automatically,
    according to 95nfs/module-setup.sh check().
    
    So, we can safely remove all the add_dracut_module "nfs".
    
    Acked-by: Dave Young <dyoung@redhat.com>
    Signed-off-by: Xunlei Pang <xlpang@redhat.com>