Get rid of the unused native code StringAddStub.

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index d4e29f67054c5263bb6a542587eec1d9aa8b332d..49f7b64f14e6b359302833d3f2be19f9087443e9 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -2228,12 +2228,8 @@ LInstruction* LChunkBuilder::DoStoreNamedGeneric(HStoreNamedGeneric* 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);
 }