Minor change on threading.Thread.native_id documentation. (GH-18129)

Remove duplication in `threading.Thread.native_id` documentation, so resulting documentation is more consistent with the `threading.Thread.ident`.

Issue initially raised [here](https://github.com/python/python-docs-fr/pull/1122#discussion_r369236634) (in French). 

No issue associated to this PR.

Automerge-Triggered-By: @csabella