Remove auto-cleanup of intents on publish

Now that we're setting a minimum cache TTL of 5 seconds, there's no
pressing need to clean these up immediately.

This behaviour previously meant that it was important that any "coming
soon' content item was published before the intent.  Removing this
behaviour simplifies this use case, and also simplifies the code.

Intents will now only be cleaned up by the nightly cronjob.