Remove explicit interface versions of dartdoc files.

lib/html/doc/interface/StringCallback.dartdoc

Index: lib/html/doc/interface/StringCallback.dartdoc
diff --git a/lib/html/doc/interface/StringCallback.dartdoc b/lib/html/doc/interface/StringCallback.dartdoc
deleted file mode 100644
index f33a4a321e1eaef03933e7363213fbf00f72d873..0000000000000000000000000000000000000000
--- a/lib/html/doc/interface/StringCallback.dartdoc
+++ /dev/null
@@ -1,9 +0,0 @@
-// Copyright (c) 2011, 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.
-
-typedef bool StringCallback(String data);