Get rid of the unused native code StringAddStub.

src/mips/full-codegen-mips.cc

Index: src/mips/full-codegen-mips.cc
diff --git a/src/mips/full-codegen-mips.cc b/src/mips/full-codegen-mips.cc
index 1956e42886031a4247ed678b2efda91368325cc9..77708cd3674df552d05f374e1ae40e25f4dc03a3 100644
--- a/src/mips/full-codegen-mips.cc
+++ b/src/mips/full-codegen-mips.cc
@@ -3730,21 +3730,13 @@ void FullCodeGenerator::EmitStringCharAt(CallRuntime* expr) {
 void FullCodeGenerator::EmitStringAdd(CallRuntime* expr) {
   ZoneList<Expression*>* args = expr->arguments();
   ASSERT_EQ(2, args->length());
-  if (FLAG_new_string_add) {
-    VisitForStackValue(args->at(0));
-    VisitForAccumulatorValue(args->at(1));
-
-    __ pop(a1);
-    __ mov(a0, result_register());  // NewStringAddStub requires args in a0, a1.
-    NewStringAddStub stub(STRING_ADD_CHECK_BOTH, NOT_TENURED);
-    __ CallStub(&stub);
-  } else {
-    VisitForStackValue(args->at(0));
-    VisitForStackValue(args->at(1));
+  VisitForStackValue(args->at(0));
+  VisitForAccumulatorValue(args->at(1));
 
-    StringAddStub stub(STRING_ADD_CHECK_BOTH);
-    __ CallStub(&stub);
-  }
+  __ pop(a1);
+  __ mov(a0, result_register());  // StringAddStub requires args in a0, a1.
+  StringAddStub stub(STRING_ADD_CHECK_BOTH, NOT_TENURED);
+  __ CallStub(&stub);
   context()->Plug(v0);
 }