Remove redundant echo.mojom, verify non-nullables in echo_service.mojom

mojo/services/dbus_echo/echo.mojom

Index: mojo/services/dbus_echo/echo.mojom
diff --git a/mojo/services/dbus_echo/echo.mojom b/mojo/services/dbus_echo/echo.mojom
deleted file mode 100644
index 947df297a4b501abd0e1f7970fcd38ff6f19cc1a..0000000000000000000000000000000000000000
--- a/mojo/services/dbus_echo/echo.mojom
+++ /dev/null
@@ -1,11 +0,0 @@
-// Copyright 2014 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 mojo {
-
-interface EchoService {
-  Echo(string? to_echo) => (string? echoed);
-};
-
-}