Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/priority_preference_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 priority preferences.
 
 // 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;
 
 import "preference_specifics.proto";
 
 // Properties of a synced priority preference.
 message PriorityPreferenceSpecifics {
   optional PreferenceSpecifics preference = 1;
 }