Fix user document deletion; setup now repeatable.

The user deletion code now properly queries the `_users` database for the proper
`_rev` value. With that value in hand, user documents are now deleted properly.

Note that the deletion is driven from the `_config/admins` list, rather than
from the list of user documents: if you have admins `{ A, B, C, D }` and users
`{ B, C, D, E }`, you'll have admins `{}` and users `{ E }` post-deletion.