Omit home_dir from account hieradata

These aren't required. The `account` module will default to
`/home/${username}` if nothing is provided, which matches the current
values:

https://github.com/alphagov/puppet-account/blob/bcd1a5f3600478a4f914137e935185a498f59a79/manifests/init.pp#L107-108

By not providing these we reduce the risk of typos and differences between
accounts.