Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/test.proto

 // Copyright (c) 2012 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.
 //
 // Protocol messages used only for testing.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
-option retain_unknown_fields = true;
 
 package sync_pb;
 
 message UnknownFieldsTestA {
   required bool foo = 1;
 }
 
 message UnknownFieldsTestB {
   required bool foo = 1;
   required bool bar = 2;
 }