Iris - Ensure hit when destroying a torn off actor from AActor::TornOff

As of CL 42014318, StopReplication requests that occur during ReceiveUpdate are delayed until all received data from the update is finished processing, including if the actor is torn off.
However, if the client destroys the actor from the TornOff function, this results in another call to StopReplicatingNetRefHandle, triggering an ensure due to this function getting called multiple times with different flags.

• Override AActor::TornOff, so that it destroys the newly torn off actor on the client
• With Iris enabled, tear off the actor on the server
• Observe the following ensure is triggered on the client:
  "Received multiple StopReplicating calls for <Actor> with different EndReplicationFlags..."