[#165] Possible incompatibility with .NET 5.0

The SslStream starts calling the inner stream's Flush method in some cases. Removed the exception since everything has been sent to the transport already.

src/Transport/TransportStream.cs

@@ -53,7 +53,6 @@ public override long Position
 
         public override void Flush()
         {
-            throw new InvalidOperationException();
         }
 
         public override int Read(byte[] buffer, int offset, int count)