remove non-working admin-migrate app, fix #26

If it doesn’t work, we drop it. We can reintroduce it as soon as it works.

An export without symmetric and _working_ import just wrongly leads people to believe they can restore their data. And then issues like @srf21c describe happen – this is exactly the quality problem we always talk about.

Please review @tomneedham @DeepDiver1975 @karlitschek @MTRichards 

Btw, does user_migrate have the same problem?