bulk update requests: fix reference to title attribute.

Also remove non-nullable and default options from migration. The column
didn't originally have these options so they shouldn't be added if the
migration is reverted.

db/migrate/20200318224633_drop_title_from_bulk_update_requests.rb

@@ -1,5 +1,5 @@
 class DropTitleFromBulkUpdateRequests < ActiveRecord::Migration[6.0]
   def change
-    remove_column :bulk_update_requests, :title, :text, null: false, default: ""
+    remove_column :bulk_update_requests, :title, :text
   end
 end