[builtins] Remove STRING_ADD_LEFT and STRING_ADD_RIGHT builtins.

src/runtime/runtime-strings.cc

Index: src/runtime/runtime-strings.cc
diff --git a/src/runtime/runtime-strings.cc b/src/runtime/runtime-strings.cc
index 33625a1b40fed5f3c2531a1ed83cd414d22480d1..0e6bf8308c3bd47ae19acdac9faeead3e4920330 100644
--- a/src/runtime/runtime-strings.cc
+++ b/src/runtime/runtime-strings.cc
@@ -1322,12 +1322,6 @@ RUNTIME_FUNCTION(Runtime_StringCharCodeAt) {
 }
 
 
-RUNTIME_FUNCTION(Runtime_IsStringWrapperSafeForDefaultValueOf) {
-  UNIMPLEMENTED();
-  return NULL;
-}
-
-
 RUNTIME_FUNCTION(Runtime_StringGetLength) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 1);