[dart:io] Remove DART_IO_DISABLED

runtime/bin/process_linux.cc

 // Copyright (c) 2012, 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/process.h"
 
 #include <errno.h>         // NOLINT
 #include <fcntl.h>         // NOLINT
 #include <poll.h>          // NOLINT
 #include <stdio.h>         // NOLINT
 #include <stdlib.h>        // NOLINT
@@ skipping 980 matching lines @@
     bzero(&act, sizeof(act));
     act.sa_handler = SIG_DFL;
     sigaction(signal, &act, NULL);
   }
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(HOST_OS_LINUX)
-
-#endif  // !defined(DART_IO_DISABLED)