Delete bogus cached previews while updating

fixes https://github.com/owncloud/core/issues/10521

To test this:
- checkout ownCloud 7
- make sure your config.php's ```version``` is bigger (or equal) than ```7.0.0.0``` and smaller (or equal) than ```7.0.2.2```
- create users
- upload files and make sure /userdir/thumbnails contains smth
- upgrade
- check if all /userdir/thumbnails folders were deleted

please review @MorrisJobke @PVince81