remove non-working admin- and user-migrate apps, fix #26

If it doesn’t work, we drop it. We can reintroduce it as soon as it works. For reference, this is the year-old bug report https://github.com/owncloud/apps/issues/26

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 

And yes, I still do believe it’s very important to have a **working** export and import.