IClientChannel.UnknownMessageReceived doesn't do anything

This API was originally intended to provide an event handler for when WCF receives a message it doesn't know how to handle, e.g. when using a duplex channel and the remote host sends a message the client can't dispatch. I believe this is intended to function the same way as ServiceHostBase.UnknownMessageReceived behaves, but for the client.
This api doesn't do anything on .NET Framework either. We should either connect it to the callback dispatcher like originally intended, or remove the api.