[dart:io] Remove DART_IO_DISABLED

runtime/bin/socket_linux.cc

 // Copyright (c) 2013, 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)
-
 #include "platform/globals.h"
 #if defined(HOST_OS_LINUX)
 
 #include "bin/socket.h"
 
 #include <errno.h>  // NOLINT
 
 #include "bin/fdutils.h"
 #include "platform/signal_blocker.h"
 
@@ skipping 158 matching lines @@
       return -1;
     }
   }
   return socket;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(HOST_OS_LINUX)
-
-#endif  // !defined(DART_IO_DISABLED)