Revert of Internalize strings in-place

src/factory.h

Index: src/factory.h
diff --git a/src/factory.h b/src/factory.h
index c79edcd27e68c1b353faa0d646f36f9626be27e7..49008e2da6ea2e94719acf5ba87df8257747c3f6 100644
--- a/src/factory.h
+++ b/src/factory.h
@@ -227,11 +227,6 @@
   MUST_USE_RESULT MaybeHandle<Map> InternalizedStringMapForString(
       Handle<String> string);
 
-  // Creates an internalized copy of an external string. |string| must be
-  // of type StringClass.
-  template <class StringClass>
-  Handle<StringClass> InternalizeExternalString(Handle<String> string);
-
   // Allocates and partially initializes an one-byte or two-byte String. The
   // characters of the string are uninitialized. Currently used in regexp code
   // only, where they are pretenured.