Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/extension_setting_specifics.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.
 //
 // Sync protocol datatype extension for an extension setting.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
-option retain_unknown_fields = true;
 
 package sync_pb;
 
 // Properties of extension setting sync objects.
 message ExtensionSettingSpecifics {
   // Id of the extension the setting is for.
   optional string extension_id = 1;
 
   // Setting key.
   optional string key = 2;
 
   // Setting value serialized as JSON.
   optional string value = 3;
 }