Nothing used the `ptree` feature or optional dependency within
`nu-command` except to include it within the `version` output. This
may be related to when `nu-cli` also had a `ptree` feature, but
I'm not sure.
That leaves the code within `nu_plugin_tree` as the sole remaining
user of `ptree`, which is already covered by the feature `tree`
and included in the `version` output.