Remove formatters helper mixin

This is for #1506.

A few comments:

* We could refactor `format_duration` into a private helper method of `SummaryNotification` (as it's the only caller now), but I'm not sure if that gains us anything.  We may want to be able to format a duration in another place and having it on the helpers like this seems OK to me.
* There weren't good opportunities to move the other helpers (`format_seconds` or `pluralize`) into notifications. `pluralize` is used when the profile is dumped and also by the deprecation formatter in its summary output. Neither context has a notification object readily available.  `format_seconds` is used from the profile output, and again, there's no notification object readily available there.  We may have more opportunities here once #1504 is done.
* Should we provide any deprecations for the `Helper` module change?  The methods are labelled `@api private` so it's probably OK not to.

/cc @JonRowe