Use zsh's default for ':completion:*:hosts'

The manual configuration of ':completion:*:hosts' causes some problems
(e.g. issue #1337), and misses useful information (especially from
~/.ssh/known_hosts; issue #690 and issue #1009).

The best option appears to be using zsh's default configuration for
':completion:*:hosts'.

While rebasing this for a new pull request (#1498 got closed by accident
and then requests to re-open it got ignored), it also drops the `users
off` setting: completion of user names can be useful, and the commit
adding it (c4434d2d) does not state why.