Revert "Handlify concat string and substring."

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index a9316f2d74bd7e7d52caff8b81c9e3283f11bd8f..e7586f5db63e11d8fb18064c7fed75b91eb5174f 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -8522,6 +8522,11 @@ class String: public Name {
   // ASCII and two byte string types.
   bool MarkAsUndetectable();
 
+  // Return a substring.
+  MUST_USE_RESULT MaybeObject* SubString(int from,
+                                         int to,
+                                         PretenureFlag pretenure = NOT_TENURED);
+
   // String equality operations.
   inline bool Equals(String* other);
   bool IsUtf8EqualTo(Vector<const char> str, bool allow_prefix_match = false);