[runtime] Remove obsolete %_StringAdd intrinsic.

src/full-codegen/arm64/full-codegen-arm64.cc

Index: src/full-codegen/arm64/full-codegen-arm64.cc
diff --git a/src/full-codegen/arm64/full-codegen-arm64.cc b/src/full-codegen/arm64/full-codegen-arm64.cc
index 8521a842ac37b7cf4207c5b31028cab03ffa87a5..d43469809463d218178a7f5687ba01d06fde20a6 100644
--- a/src/full-codegen/arm64/full-codegen-arm64.cc
+++ b/src/full-codegen/arm64/full-codegen-arm64.cc
@@ -3543,22 +3543,6 @@ void FullCodeGenerator::EmitStringCharAt(CallRuntime* expr) {
 }
 
 
-void FullCodeGenerator::EmitStringAdd(CallRuntime* expr) {
-  ASM_LOCATION("FullCodeGenerator::EmitStringAdd");
-  ZoneList<Expression*>* args = expr->arguments();
-  DCHECK_EQ(2, args->length());
-
-  VisitForStackValue(args->at(0));
-  VisitForAccumulatorValue(args->at(1));
-
-  __ Pop(x1);
-  StringAddStub stub(isolate(), STRING_ADD_CHECK_BOTH, NOT_TENURED);
-  __ CallStub(&stub);
-
-  context()->Plug(x0);
-}
-
-
 void FullCodeGenerator::EmitCall(CallRuntime* expr) {
   ASM_LOCATION("FullCodeGenerator::EmitCall");
   ZoneList<Expression*>* args = expr->arguments();