libct/specconv/CreateCgroupConfig: don't set c.Parent default

c.Parent is only used by systemd cgroup drivers, and both v1 and v2
drivers do have code to set the default if it is empty, so setting
it here is redundant.

In addition, in case of cgroup v2 rootless container setting it here
is harmful as the default should be user.slice not system.slice.

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

libcontainer/specconv/spec_linux_test.go

@@ -345,7 +345,7 @@ func TestLinuxCgroupSystemdWithEmptyPath(t *testing.T) {
 		t.Errorf("Couldn't create Cgroup config: %v", err)
 	}
 
-	expectedParent := "system.slice"
+	expectedParent := ""
 	if cgroup.Parent != expectedParent {
 		t.Errorf("Expected to have %s as Parent instead of %s", expectedParent, cgroup.Parent)
 	}