Migration to remove all non-Maslow needs from artefacts

Some artefacts have old need_ids which predate Maslow (these are ones
that aren't 6 digits). There old needs are either in Need-o-tron or
in spreadsheets. All old needs that were salvageable have now been
imported into Maslow and the artefacts retagged with the new Maslow need
through the Panopticon UI.

All remaining non-Maslow need_ids can now be removed.
It would be nice to print out the need ids that are being removed from artefacts as this goes.  This would make it easier to spot if any ones are being removed that we didn't mean to remove (eg, due to an error in the regex or similar problem).

I'd suggest then saving the output log when this is run in production, so that if we needed to unpick some of the changes we could do that manually.