Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/article_specifics.proto

 // Copyright 2013 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.
 //
 // Sync protocol datatype extension for the article.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
-option retain_unknown_fields = true;
 
 package sync_pb;
 
 import "attachments.proto";
 
 // Properties of Article objects.
 message ArticleSpecifics {
   optional string entry_id = 1;
 
   optional string title = 2;
 
   repeated ArticlePage pages = 3;
 
   optional ArticleAttachments attachments = 4;
 }
 
 message ArticlePage {
   optional string url = 1;
 }
 
 message ArticleAttachments {
   optional AttachmentIdProto distilled_article = 1;
 }