Get rid of the unused native code StringAddStub.

src/mips/lithium-mips.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 2187 matching lines @@
   LOperand* obj = UseFixed(instr->object(), a1);
   LOperand* val = UseFixed(instr->value(), a0);
 
   LInstruction* result = new(zone()) LStoreNamedGeneric(context, obj, val);
   return MarkAsCall(result, instr);
 }
 
 
 LInstruction* LChunkBuilder::DoStringAdd(HStringAdd* instr) {
   LOperand* context = UseFixed(instr->context(), cp);
-  LOperand* left = FLAG_new_string_add
-      ? UseFixed(instr->left(), a1)
-      : UseRegisterAtStart(instr->left());
-  LOperand* right = FLAG_new_string_add
-      ? UseFixed(instr->right(), a0)
-      : UseRegisterAtStart(instr->right());
+  LOperand* left = UseFixed(instr->left(), a1);
+  LOperand* right = UseFixed(instr->right(), a0);
   return MarkAsCall(
       DefineFixed(new(zone()) LStringAdd(context, left, right), v0),
       instr);
 }
 
 
 LInstruction* LChunkBuilder::DoStringCharCodeAt(HStringCharCodeAt* instr) {
   LOperand* string = UseTempRegister(instr->string());
   LOperand* index = UseTempRegister(instr->index());
   LOperand* context = UseAny(instr->context());
@@ skipping 235 matching lines @@
 
 
 LInstruction* LChunkBuilder::DoLoadFieldByIndex(HLoadFieldByIndex* instr) {
   LOperand* object = UseRegister(instr->object());
   LOperand* index = UseRegister(instr->index());
   return DefineAsRegister(new(zone()) LLoadFieldByIndex(object, index));
 }
 
 
 } }  // namespace v8::internal