Get rid of the unused native code StringAddStub.

src/ia32/lithium-ia32.cc

Index: src/ia32/lithium-ia32.cc
diff --git a/src/ia32/lithium-ia32.cc b/src/ia32/lithium-ia32.cc
index b37589397fc94b4d9853fd7a6472feedb487fa9d..b4b44521cd0c0a4dd3227a455b3683f0b4b6f2fa 100644
--- a/src/ia32/lithium-ia32.cc
+++ b/src/ia32/lithium-ia32.cc
@@ -2378,12 +2378,8 @@ LInstruction* LChunkBuilder::DoStoreNamedGeneric(HStoreNamedGeneric* instr) {
 
 LInstruction* LChunkBuilder::DoStringAdd(HStringAdd* instr) {
   LOperand* context = UseFixed(instr->context(), esi);
-  LOperand* left = FLAG_new_string_add
-      ? UseFixed(instr->left(), edx)
-      : UseOrConstantAtStart(instr->left());
-  LOperand* right = FLAG_new_string_add
-      ? UseFixed(instr->right(), eax)
-      : UseOrConstantAtStart(instr->right());
+  LOperand* left = UseFixed(instr->left(), edx);
+  LOperand* right = UseFixed(instr->right(), eax);
   LStringAdd* string_add = new(zone()) LStringAdd(context, left, right);
   return MarkAsCall(DefineFixed(string_add, eax), instr);
 }