removeHandler(context:removalToken:)

Ask the receiving RemovableChannelHandler to remove itself from the ChannelPipeline as soon as possible. The receiving RemovableChannelHandler may elect to remove itself sometime after this method call, rather than immediately, but if it does so it must take the necessary precautions to handle events arriving between the invocation of this method and the call to ChannelHandlerContext.leavePipeline that triggers the actual removal.