Get rid of the unused native code StringAddStub.

src/x64/lithium-x64.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2210 matching lines @@
   LOperand* value = UseFixed(instr->value(), rax);
 
   LStoreNamedGeneric* result =
       new(zone()) LStoreNamedGeneric(context, object, value);
   return MarkAsCall(result, instr);
 }
 
 
 LInstruction* LChunkBuilder::DoStringAdd(HStringAdd* instr) {
   LOperand* context = UseFixed(instr->context(), rsi);
-  LOperand* left = FLAG_new_string_add
-      ? UseFixed(instr->left(), rdx)
-      : UseOrConstantAtStart(instr->left());
-  LOperand* right = FLAG_new_string_add
-      ? UseFixed(instr->right(), rax)
-      : UseOrConstantAtStart(instr->right());
+  LOperand* left = UseFixed(instr->left(), rdx);
+  LOperand* right = UseFixed(instr->right(), rax);
   return MarkAsCall(
       DefineFixed(new(zone()) LStringAdd(context, left, right), rax), instr);
 }
 
 
 LInstruction* LChunkBuilder::DoStringCharCodeAt(HStringCharCodeAt* instr) {
   LOperand* string = UseTempRegister(instr->string());
   LOperand* index = UseTempRegister(instr->index());
   LOperand* context = UseAny(instr->context());
   LStringCharCodeAt* result =
@@ skipping 246 matching lines @@
 LInstruction* LChunkBuilder::DoLoadFieldByIndex(HLoadFieldByIndex* instr) {
   LOperand* object = UseRegister(instr->object());
   LOperand* index = UseTempRegister(instr->index());
   return DefineSameAsFirst(new(zone()) LLoadFieldByIndex(object, index));
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64