child_process: remove dummy "setsid" option setting

doc/api/child_process.markdown

@@ -133,15 +133,12 @@ If omitted, `args` defaults to an empty Array.
 The third argument is used to specify additional options, which defaults to:
 
     { cwd: undefined,
-      env: process.env,
-      setsid: false
+      env: process.env
     }
 
 `cwd` allows you to specify the working directory from which the process is spawned.
 Use `env` to specify environment variables that will be visible to the new process.
 
-`setsid`, if set true, will cause the subprocess to be run in a new session.
-
 Example of running `ls -lh /usr`, capturing `stdout`, `stderr`, and the exit code:
 
     var util  = require('util'),

lib/child_process.js

@@ -248,7 +248,6 @@ exports.execFile = function(file /* args, options, callback */) {
     timeout: 0,
     maxBuffer: 200 * 1024,
     killSignal: 'SIGTERM',
-    setsid: false,
     cwd: null,
     env: null
   };