runtime: 2 members on WebSocket throw PNSE

These are both part of NS2.0. Both show no usage in our data. Please triage to 2.0, future, or close.

        public static void RegisterPrefixes()
        {
            // The current WebRequest implementation in corefx does not support upgrading
            // web socket connections.  For now, we throw.
            throw new PlatformNotSupportedException();
        }

        [EditorBrowsable(EditorBrowsableState.Never)]
        public static WebSocket CreateClientWebSocket(Stream innerStream,
            string subProtocol, int receiveBufferSize, int sendBufferSize,
            TimeSpan keepAliveInterval, bool useZeroMaskingKey, ArraySegment<byte> internalBuffer)
        {
            // ClientWebSocket on Unix is implemented in managed code and can be constructed (internally)
            // for an arbitrary stream. We could use that implementation here, building it in to the WebSocket
            // library as well, or accessing it from the client library via reflection.  For now, we throw.
            throw new PlatformNotSupportedException();
        }