Headless: Remove obsolete Mojo code and some other obsolete APIs

headless/lib/embedder_test.mojom

Index: headless/lib/embedder_test.mojom
diff --git a/headless/lib/embedder_test.mojom b/headless/lib/embedder_test.mojom
deleted file mode 100644
index 553709a198651778898a7dd081d802896f1b81cb..0000000000000000000000000000000000000000
--- a/headless/lib/embedder_test.mojom
+++ /dev/null
@@ -1,11 +0,0 @@
-// Copyright 2016 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-module embedder_test;
-
-// An example embedder Mojo service.
-interface TestEmbedderService {
-  // Returns a test result to the embedder.
-  ReturnTestResult(string result);
-};