Remove explicit interface versions of dartdoc files.

lib/html/doc/interface/CanvasGradient.dartdoc

Index: lib/html/doc/interface/CanvasGradient.dartdoc
diff --git a/lib/html/doc/interface/CanvasGradient.dartdoc b/lib/html/doc/interface/CanvasGradient.dartdoc
deleted file mode 100644
index 56e41f14c9ff1e5ade80bc530b30d5772c48fafc..0000000000000000000000000000000000000000
--- a/lib/html/doc/interface/CanvasGradient.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 CanvasGradient
-interface CanvasGradient {
-
-  /** @domName CanvasGradient.addColorStop */
-  void addColorStop(num offset, String color);
-}