[builtins] Remove STRING_ADD_LEFT and STRING_ADD_RIGHT builtins.

src/runtime/runtime-strings.cc

 // Copyright 2014 the V8 project 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 "src/runtime/runtime-utils.h"
 
 #include "src/arguments.h"
 #include "src/conversions-inl.h"
 #include "src/isolate-inl.h"
 #include "src/regexp/jsregexp-inl.h"
@@ skipping 1304 matching lines @@
 RUNTIME_FUNCTION(Runtime_StringCharCodeAt) {
   SealHandleScope shs(isolate);
   DCHECK(args.length() == 2);
   if (!args[0]->IsString()) return isolate->heap()->undefined_value();
   if (!args[1]->IsNumber()) return isolate->heap()->undefined_value();
   if (std::isinf(args.number_at(1))) return isolate->heap()->nan_value();
   return __RT_impl_Runtime_StringCharCodeAtRT(args, isolate);
 }
 
 
-RUNTIME_FUNCTION(Runtime_IsStringWrapperSafeForDefaultValueOf) {
-  UNIMPLEMENTED();
-  return NULL;
-}
-
-
 RUNTIME_FUNCTION(Runtime_StringGetLength) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 1);
   CONVERT_ARG_HANDLE_CHECKED(String, s, 0);
   return Smi::FromInt(s->length());
 }
 }  // namespace internal
 }  // namespace v8