[runtime] Remove obsolete %_StringAdd intrinsic.

src/full-codegen/mips/full-codegen-mips.cc

Index: src/full-codegen/mips/full-codegen-mips.cc
diff --git a/src/full-codegen/mips/full-codegen-mips.cc b/src/full-codegen/mips/full-codegen-mips.cc
index c90731de15108d697c09babe325d523f3fa6ce8f..9719238d8271d22ea5ac69b8c2b9803752db3a6d 100644
--- a/src/full-codegen/mips/full-codegen-mips.cc
+++ b/src/full-codegen/mips/full-codegen-mips.cc
@@ -3848,20 +3848,6 @@ 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(a1);
-  __ mov(a0, result_register());  // StringAddStub requires args in a0, a1.
-  StringAddStub stub(isolate(), STRING_ADD_CHECK_BOTH, NOT_TENURED);
-  __ CallStub(&stub);
-  context()->Plug(v0);
-}
-
-
 void FullCodeGenerator::EmitCall(CallRuntime* expr) {
   ZoneList<Expression*>* args = expr->arguments();
   DCHECK_LE(2, args->length());