[CS0420] ActorCell Warning Disable (#6683)

* [CS0420] `ActorCell` Warning Disable

* [fix] Removed the `Volatile.Read`

Author: eaba

src/core/Akka/Actor/ActorCell.cs

@@ -91,7 +91,7 @@ public ActorCell(ActorSystemImpl system, IInternalActorRef self, Props props, Me
         /// <summary>
         /// TBD
         /// </summary>
-        public Mailbox Mailbox => Volatile.Read(ref _mailboxDoNotCallMeDirectly);
+        public Mailbox Mailbox => _mailboxDoNotCallMeDirectly;
 
         /// <summary>
         /// TBD