fix yo communication channel deletion migration

Change-Id: Icf20cb7ad8767d0b819f481d0e5e52dc8a8e3d74
Reviewed-on: https://gerrit.instructure.com/147789
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>
Tested-by: Jenkins

db/migrate/20180408035827_delete_yo_communication_channels.rb

@@ -20,6 +20,7 @@ class DeleteYoCommunicationChannels < ActiveRecord::Migration[5.1]
   tag :postdeploy
 
   def up
+    NotificationPolicy.where(:communication_channel_id => CommunicationChannel.where(path_type: 'yo').select(:id)).delete_all
     CommunicationChannel.where(path_type: 'yo').delete_all
   end
 end