Remove ptree dep from nu-command, remove associated feature. (#3741)

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.

crates/nu-command/src/commands/core_commands/version.rs

@@ -191,11 +191,6 @@ fn features_enabled() -> Vec<String> {
         names.push("ctrlc".to_string());
     }
 
-    #[cfg(feature = "ptree")]
-    {
-        names.push("ptree".to_string());
-    }
-
     // #[cfg(feature = "rich-benchmark")]
     // {
     //     names.push("rich-benchmark".to_string());