Get rid of the unused native code StringAddStub.

src/mips/code-stubs-mips.h

Index: src/mips/code-stubs-mips.h
diff --git a/src/mips/code-stubs-mips.h b/src/mips/code-stubs-mips.h
index 3019220e7fd26c0c3177cbb7266e650ee09498a1..1ee3127e052b24b1e5bd0b8475dc6354c603a1a4 100644
--- a/src/mips/code-stubs-mips.h
+++ b/src/mips/code-stubs-mips.h
@@ -120,32 +120,6 @@ class StringHelper : public AllStatic {
 };
 
 
-class StringAddStub: public PlatformCodeStub {
- public:
-  explicit StringAddStub(StringAddFlags flags) : flags_(flags) {}
-
- private:
-  Major MajorKey() { return StringAdd; }
-  int MinorKey() { return flags_; }
-
-  void Generate(MacroAssembler* masm);
-
-  void GenerateConvertArgument(MacroAssembler* masm,
-                               int stack_offset,
-                               Register arg,
-                               Register scratch1,
-                               Register scratch2,
-                               Register scratch3,
-                               Register scratch4,
-                               Label* slow);
-
-  void GenerateRegisterArgsPush(MacroAssembler* masm);
-  void GenerateRegisterArgsPop(MacroAssembler* masm);
-
-  const StringAddFlags flags_;
-};
-
-
 class SubStringStub: public PlatformCodeStub {
  public:
   SubStringStub() {}