fixes the `block_removed_ldap_users` rake task

In e23a26a (and later 1bc9936) the API for Gitlab::LDAP::Adapter was
changed. I assume this rake task was an oversight in the refactoring of
the changed class.

While being on it, I noticed that already blocked users cannot be
blocked again.
Merge pull request #8291 from MrMarvin/fix/ldap_cleanup_task

fixes the `block_removed_ldap_users` rake task