Remove libcgroup

Nothing seems to use it.

It was introduced as docker-ce dependency in
kubernetes#9465, however, docker's RPMs never
depended on it.

I went through
https://github.com/docker/docker-ce-packaging/tree/360f755c768ebb06c9974705674302098bb9c398/rpm
(referred by the PR above) and today's RPMs in
https://github.com/docker/docker-ce-packaging/tree/master/rpm, they don't
require libcgroup.

nodeup/pkg/model/packages.go

@@ -61,7 +61,6 @@ func (b *PackagesBuilder) Build(c *fi.NodeupModelBuilderContext) error {
 		c.AddTask(&nodetasks.Package{Name: "ebtables"})
 		c.AddTask(&nodetasks.Package{Name: "ethtool"})
 		c.AddTask(&nodetasks.Package{Name: "iptables"})
-		c.AddTask(&nodetasks.Package{Name: "libcgroup"})
 		c.AddTask(&nodetasks.Package{Name: "libseccomp"})
 		c.AddTask(&nodetasks.Package{Name: "libtool-ltdl"})
 		c.AddTask(&nodetasks.Package{Name: "socat"})