Remove explicit interface versions of dartdoc files.

lib/html/doc/interface/HeadingElement.dartdoc

Index: lib/html/doc/interface/HeadingElement.dartdoc
diff --git a/lib/html/doc/interface/HeadingElement.dartdoc b/lib/html/doc/interface/HeadingElement.dartdoc
deleted file mode 100644
index 9a5f1bb025920492bd6ba9aa684edf09d8001269..0000000000000000000000000000000000000000
--- a/lib/html/doc/interface/HeadingElement.dartdoc
+++ /dev/null
@@ -1,26 +0,0 @@
-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-// WARNING:
-//   This file contains documentation that is merged into the real source.
-//   Do not make code changes here.
-
-/// @domName HTMLHeadingElement
-interface HeadingElement extends Element default _Elements {
-
-  HeadingElement.h1();
-
-  HeadingElement.h2();
-
-  HeadingElement.h3();
-
-  HeadingElement.h4();
-
-  HeadingElement.h5();
-
-  HeadingElement.h6();
-
-  /** @domName HTMLHeadingElement.align */
-  String align;
-}