Do not cancel PuSH subscriptions after encountering "permanent" error… (#3046)

* Do not cancel PuSH subscriptions after encountering "permanent" error response

After talking with MMN about it, turns out some servers/php setups do
return 4xx errors while rebooting, so this anti-feature that was meant
to take load off of the hub is doing more harm than good in terms of
breaking subscriptions

* Update delivery_worker.rb