[dart:io] Remove DART_IO_DISABLED

runtime/bin/process_linux.cc

Index: runtime/bin/process_linux.cc
diff --git a/runtime/bin/process_linux.cc b/runtime/bin/process_linux.cc
index cbaa0128388a39688fdfa9826bd7f3e7bc8e4a8a..2431f0f61652516f78125c52843a4f7439c6e027 100644
--- a/runtime/bin/process_linux.cc
+++ b/runtime/bin/process_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)
 
@@ -1004,5 +1002,3 @@ void Process::ClearSignalHandler(intptr_t signal, Dart_Port port) {
 }  // namespace dart
 
 #endif  // defined(HOST_OS_LINUX)
-
-#endif  // !defined(DART_IO_DISABLED)