Fix regression in JSON serialization of RegExps.

src/string.js

Index: src/string.js
diff --git a/src/string.js b/src/string.js
index 3b3c82bb7214e697c7845153c0eab73ec79a7f39..850399f44a0a397bea41669a0392163d4f2ae86e 100644
--- a/src/string.js
+++ b/src/string.js
@@ -875,11 +875,6 @@ ReplaceResultBuilder.prototype.generate = function() {
 }
 
 
-function StringToJSON(key) {
-  return CheckJSONPrimitive(this.valueOf());
-}
-
-
 // -------------------------------------------------------------------
 
 function SetupString() {
@@ -929,8 +924,7 @@ function SetupString() {
     "small", StringSmall,
     "strike", StringStrike,
     "sub", StringSub,
-    "sup", StringSup,
-    "toJSON", StringToJSON
+    "sup", StringSup
   ));
 }