[dart:io] Remove DART_IO_DISABLED

runtime/bin/sync_socket_macos.cc

Index: runtime/bin/sync_socket_macos.cc
diff --git a/runtime/bin/sync_socket_macos.cc b/runtime/bin/sync_socket_macos.cc
index 61df267250e2241523ef87c470acc1ffc326c96f..5ee9b68c81f066ca05c3d1045ee1439b5b30bc42 100644
--- a/runtime/bin/sync_socket_macos.cc
+++ b/runtime/bin/sync_socket_macos.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_MACOS)
 
@@ -95,5 +93,3 @@ void SynchronousSocket::Close(intptr_t fd) {
 }  // namespace dart
 
 #endif  // defined(HOST_OS_MACOS)
-
-#endif  // !defined(DART_IO_DISABLED)