cgroupv2: don't use getSubsystemPath in Apply

This code is a copy-paste from cgroupv1 systemd code. Its aim
is to check whether a subsystem is available, and skip those
that are not.

In case v2 unified hierarchy is used, getSubsystemPath never
returns "not found" error, so calling it is useless.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/systemd/unified_hierarchy.go

@@ -38,14 +38,6 @@ func (m *UnifiedManager) Apply(pid int) error {
 	if c.Paths != nil {
 		paths := make(map[string]string)
 		for name, path := range c.Paths {
-			_, err := getSubsystemPath(m.Cgroups, name)
-			if err != nil {
-				// Don't fail if a cgroup hierarchy was not found, just skip this subsystem
-				if cgroups.IsNotFound(err) {
-					continue
-				}
-				return err
-			}
 			paths[name] = path
 		}
 		m.Paths = paths