Remove kChildCleanExit since it's not used anymore.

content/public/common/content_switches.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/public/common/content_switches.h"
 
 #include "base/command_line.h"
 
 namespace switches {
 
@@ skipping 1029 matching lines @@
 
 #if defined(OS_MACOSX) && !defined(OS_IOS)
 const char kEnableCarbonInterposing[]       = "enable-carbon-interposing";
 
 // Disables support for Core Animation plugins. This is triggered when
 // accelerated compositing is disabled. See http://crbug.com/122430 .
 const char kDisableCoreAnimationPlugins[] =
     "disable-core-animation-plugins";
 #endif
 
-#if defined(OS_POSIX)
-// Causes the child processes to cleanly exit via calling exit().
-const char kChildCleanExit[]                = "child-clean-exit";
-#endif
-
 // Don't dump stuff here, follow the same order as the header.
 
 }  // namespace switches