[runtime] Remove obsolete %_StringAdd intrinsic.

src/full-codegen/x64/full-codegen-x64.cc

Index: src/full-codegen/x64/full-codegen-x64.cc
diff --git a/src/full-codegen/x64/full-codegen-x64.cc b/src/full-codegen/x64/full-codegen-x64.cc
index 9420127a48fac0b81063fbc6550ae9abd3e69743..ecc3d405b32a25c872a67720c2f9092b6bb62678 100644
--- a/src/full-codegen/x64/full-codegen-x64.cc
+++ b/src/full-codegen/x64/full-codegen-x64.cc
@@ -3729,19 +3729,6 @@ void FullCodeGenerator::EmitStringCharAt(CallRuntime* expr) {
 }
 
 
-void FullCodeGenerator::EmitStringAdd(CallRuntime* expr) {
-  ZoneList<Expression*>* args = expr->arguments();
-  DCHECK_EQ(2, args->length());
-  VisitForStackValue(args->at(0));
-  VisitForAccumulatorValue(args->at(1));
-
-  __ Pop(rdx);
-  StringAddStub stub(isolate(), STRING_ADD_CHECK_BOTH, NOT_TENURED);
-  __ CallStub(&stub);
-  context()->Plug(rax);
-}
-
-
 void FullCodeGenerator::EmitCall(CallRuntime* expr) {
   ZoneList<Expression*>* args = expr->arguments();
   DCHECK_LE(2, args->length());