[local-sync] Don't delete messages via cascade when a folder is deleted

1. It could be expensive to delete many messages at the exact moment
   when the folder is deleted
2. The folder delete could actually just be a rename, and if we
   deleted all the messages, we would have to re-process them all
3. We already do a clean-up check for orphaned messages at the end
   of the sync loop, where we already know if the folder was
   actually deleted or just renamed

packages/local-sync/src/models/folder.js

@@ -40,7 +40,7 @@ module.exports = (sequelize, Sequelize) => {
   }, {
     classMethods: {
       associate({Folder, Message, Thread}) {
-        Folder.hasMany(Message, {onDelete: 'cascade', hooks: true})
+        Folder.hasMany(Message)
         Folder.belongsToMany(Thread, {through: 'thread_folders'})
       },