Remove unused code, AstValueFactory::ConcatStrings.

src/ast/ast-value-factory.h

Index: src/ast/ast-value-factory.h
diff --git a/src/ast/ast-value-factory.h b/src/ast/ast-value-factory.h
index 4ce480fe570b94b6fda45492592985afaeb137cb..63b077a8d21e6307e6bef4356444ea441344a1f0 100644
--- a/src/ast/ast-value-factory.h
+++ b/src/ast/ast-value-factory.h
@@ -361,8 +361,6 @@ class AstValueFactory {
   const AstRawString* GetString(Handle<String> literal);
   const AstConsString* NewConsString(const AstString* left,
                                      const AstString* right);
-  const AstRawString* ConcatStrings(const AstRawString* left,
-                                    const AstRawString* right);
 
   void Internalize(Isolate* isolate);