Get rid of the unused native code StringAddStub.

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index 50610897d6f17d4591e183c3a3494395969e45bd..fb382d13c1056c624d25d46f750dc3be08b769b6 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -3610,21 +3610,12 @@ void FullCodeGenerator::EmitStringCharAt(CallRuntime* expr) {
 void FullCodeGenerator::EmitStringAdd(CallRuntime* expr) {
   ZoneList<Expression*>* args = expr->arguments();
   ASSERT_EQ(2, args->length());
+  VisitForStackValue(args->at(0));
+  VisitForAccumulatorValue(args->at(1));
 
-  if (FLAG_new_string_add) {
-    VisitForStackValue(args->at(0));
-    VisitForAccumulatorValue(args->at(1));
-
-    __ pop(rdx);
-    NewStringAddStub stub(STRING_ADD_CHECK_BOTH, NOT_TENURED);
-    __ CallStub(&stub);
-  } else {
-    VisitForStackValue(args->at(0));
-    VisitForStackValue(args->at(1));
-
-    StringAddStub stub(STRING_ADD_CHECK_BOTH);
-    __ CallStub(&stub);
-  }
+  __ pop(rdx);
+  StringAddStub stub(STRING_ADD_CHECK_BOTH, NOT_TENURED);
+  __ CallStub(&stub);
   context()->Plug(rax);
 }