nsenter: remove cgroupns sync mechanism

As pointed out in TODO item added by commit 64bb59f59, it is not
necessary to have a special sync mechanism for cgroupns, as the parent
adds runc init to cgroup way earlier (before sending nl bootstrap data.

This sync was added by commit df3fa115f9, which was also added a
second cgroup manager.Apply() call, later removed in commit
d1ba8e39f8. It seems the original author had the idea to wait for
that second Apply().

Fixes: df3fa115f9
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/process_linux.go

@@ -25,10 +25,6 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-// Synchronisation value for cgroup namespace setup.
-// The same constant is defined in nsexec.c as "CREATECGROUPNS".
-const createCgroupns = 0x80
-
 type parentProcess interface {
 	// pid returns the pid for the running process.
 	pid() int
@@ -411,13 +407,6 @@ func (p *initProcess) start() (retErr error) {
 	}
 	p.setExternalDescriptors(fds)
 
-	// Now it's time to setup cgroup namesapce
-	if p.config.Config.Namespaces.Contains(configs.NEWCGROUP) && p.config.Config.Namespaces.PathOf(configs.NEWCGROUP) == "" {
-		if _, err := p.messageSockPair.parent.Write([]byte{createCgroupns}); err != nil {
-			return fmt.Errorf("error sending synchronization value to init process: %w", err)
-		}
-	}
-
 	// Wait for our first child to exit
 	if err := p.waitForChildExit(childPid); err != nil {
 		return fmt.Errorf("error waiting for our first child to exit: %w", err)