Remove plaintext message test case

Support for the PLAINTEXT message type is not present in the latest
protobuf definitions. Leaving it out for now since we don't have any use
case for it currently.

libtextsecure/test/index.html

@@ -50,7 +50,6 @@
   <script type="text/javascript" src="fake_api.js"></script>
   <script type="text/javascript" src="helpers_test.js"></script>
   <script type="text/javascript" src="websocket-resources_test.js"></script>
-  <script type="text/javascript" src="protocol_test.js"></script>
   <script type="text/javascript" src="storage_test.js"></script>
   <script type="text/javascript" src="generate_keys_test.js"></script>
   <script type="text/javascript" src="websocket_test.js"></script>