Update to buildtools 2.0.0-prerelease-01928-01

This brings the latest compiler to our desktop-MSBuild builds as well.

I had to fix one instance of an unused field in the networking code.
I checked and that field was present in desktop to control logging all
consuming codepaths are missing from core.

BuildToolsVersion.txt

@@ -1 +1 @@
-2.0.0-prerelease-01925-01
+2.0.0-prerelease-01928-01

src/System.Net.HttpListener/src/System/Net/Windows/WebSockets/WebSocketBase.cs

@@ -18,9 +18,6 @@ namespace System.Net.WebSockets
 {
     internal abstract class WebSocketBase : WebSocket, IDisposable
     {
-#if DEBUG
-        private volatile string _closeStack;
-#endif
         private readonly OutstandingOperationHelper _closeOutstandingOperationHelper;
         private readonly OutstandingOperationHelper _closeOutputOutstandingOperationHelper;
         private readonly OutstandingOperationHelper _receiveOutstandingOperationHelper;
@@ -35,7 +32,6 @@ internal abstract class WebSocketBase : WebSocket, IDisposable
         private readonly SemaphoreSlim _sendFrameThrottle;
         // locking _ThisLock protects access to
         // - State
-        // - _closeStack
         // - _closeAsyncStartedReceive
         // - _closeReceivedTaskCompletionSource
         // - _closeNetworkConnectionTask