Delete deprecated synced notifications code

sync/protocol/synced_notification_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 synced notifications.
-
-// Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
-// any fields in this file.
+// DO NOT USE: This datatype is deprecated.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 option retain_unknown_fields = true;
 
 package sync_pb;
 
-import "synced_notification_data.proto";
-import "synced_notification_render.proto";
-
-// Properties of SyncedNotificationSpecifics objects.
+// This message is kept around for backwards compatibility sake.
 message SyncedNotificationSpecifics {
-  // The notification from the server.
-  optional CoalescedSyncedNotification coalesced_notification = 1;
 }