This change removes two recently added hooks. No deprecation cycle
because they have only existed for a week, so it's extremely unlikely
that anyone's using them.
The hook gets fired for each product individually, so a single mtime
update is not good enough.
The timestamp needs to be recorded per product or tag. Meh.
RevBank reads the new products file on every interaction (e.g. pressing
enter), and then fires hooks like `product_changed`. Every running
instance gets those hooks, but the price tage should be generated only
once.
