[dart:io] Remove DART_IO_DISABLED

runtime/bin/sync_socket_linux.cc

Index: runtime/bin/sync_socket_linux.cc
diff --git a/runtime/bin/sync_socket_linux.cc b/runtime/bin/sync_socket_linux.cc
index 99abcffa36ac42f2360c9b58b0e0f150362d227c..1fb564bfaad09712257f70e048afc62ef29cbf09 100644
--- a/runtime/bin/sync_socket_linux.cc
+++ b/runtime/bin/sync_socket_linux.cc
@@ -2,8 +2,6 @@
 // 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)
-
 #include "platform/globals.h"
 #if defined(HOST_OS_LINUX)
 
@@ -92,5 +90,3 @@ void SynchronousSocket::Close(intptr_t fd) {
 }  // namespace dart
 
 #endif  // defined(HOST_OS_LINUX)
-
-#endif  // !defined(DART_IO_DISABLED)