[dart:io] Remove DART_IO_DISABLED

runtime/bin/security_context_linux.cc

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-#if !defined(DART_IO_DISABLED) && !defined(DART_IO_SECURE_SOCKET_DISABLED)
+#if !defined(DART_IO_SECURE_SOCKET_DISABLED)
 
 #include "platform/globals.h"
 #if defined(HOST_OS_LINUX)
 
 #include "bin/security_context.h"
 
 #include <openssl/bio.h>
 #include <openssl/ssl.h>
 #include <openssl/x509.h>
 
@@ skipping 56 matching lines @@
 void SSLCertContext::RegisterCallbacks(SSL* ssl) {
   // No callbacks to register for implementations using BoringSSL's built-in
   // verification mechanism.
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(HOST_OS_LINUX)
 
-#endif  // !defined(DART_IO_DISABLED) &&
-        // !defined(DART_IO_SECURE_SOCKET_DISABLED)
+#endif  // !defined(DART_IO_SECURE_SOCKET_DISABLED)