Remove explicit interface versions of dartdoc files.

lib/html/doc/interface/ContentElement.dartdoc

Index: lib/html/doc/interface/ContentElement.dartdoc
diff --git a/lib/html/doc/interface/ContentElement.dartdoc b/lib/html/doc/interface/ContentElement.dartdoc
deleted file mode 100644
index 7905d9bd4861d3627b67e01070da09971b05fc0a..0000000000000000000000000000000000000000
--- a/lib/html/doc/interface/ContentElement.dartdoc
+++ /dev/null
@@ -1,14 +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 HTMLContentElement
-interface ContentElement extends Element {
-
-  /** @domName HTMLContentElement.select */
-  String select;
-}