Remove DocumentParser::flush which is not used by anyone.

third_party/WebKit/Source/core/dom/RawDataDocumentParser.h

 /*
  * Copyright (C) 2010 Google, Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 27 matching lines @@
     {
     }
 
     void finish() override
     {
         if (!isStopped())
             document()->finishedParsing();
     }
 
 private:
-    void flush() override
-    {
-        // Make sure appendBytes is called at least once.
-        appendBytes(0, 0);
-    }
+    void insert(const SegmentedString&) final {}
 
-    void insert(const SegmentedString&) override
-    {
-    }
-
-    void append(const String&) override
+    void append(const String&) final
     {
         ASSERT_NOT_REACHED();
     }
 };
 
 };
 
 #endif // RawDataDocumentParser_h