libcontainerd/supervisor: remove unused RWMutex

This RWMutex was added in 9c4570a958df42d1ad19364b1a8da55b891d850a, and used in
the `remote.Client()` method. Commit dd2e19ebd58cd8896d79b4b8db61144b93717b33
split the code for client and daemon, but did not remove the mutex.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

libcontainerd/supervisor/remote_daemon.go

@@ -9,7 +9,6 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
-	"sync"
 	"time"
 
 	"github.com/containerd/containerd"
@@ -31,7 +30,6 @@ const (
 )
 
 type remote struct {
-	sync.RWMutex
 	config.Config
 
 	daemonPid int