Revert "Add temporary users for XVT"

This reverts commit b89f40efcc9cecc94a2c5a69c86a74e49122108c.
users: Simplify management of "extra" users

Remove duplicated code from users role in favour of just using
with_flattened as intended.

Also renamed extra_users to users_extra for better sortability in YAML
vars files.
users: Add a simple way of purging old users
users: Remove line noise from users role