Remove cruft that dealt with env parameter

Originally added in commit 078a48a9, this code dealt with an optional
env parameter that was passed to `exec`.  The parameter was removed, but
this code was left.  As it serves no purpose, removing it.

lib/child_process.js

@@ -14,10 +14,8 @@ var spawn = exports.spawn = function (path, args /*, options OR env, customFds *
 exports.exec = function (command /*, options, callback */) {
   if (arguments.length < 3) {
     return exports.execFile("/bin/sh", ["-c", command], arguments[1]);
-  } else if (arguments.length < 4) {
-    return exports.execFile("/bin/sh", ["-c", command], arguments[1], arguments[2]);
   } else {
-    return exports.execFile("/bin/sh", ["-c", command], arguments[1], arguments[2], arguments[3]);
+    return exports.execFile("/bin/sh", ["-c", command], arguments[1], arguments[2]);
   }
 };