doc: remove vestigial onboarding section

The onboarding-extras doc includes a section with a single bullet point
instructing the new Collaborator to write good pull request
descriptions. This material is likely superfluous and is in the wrong
document if it is not. (It more properly belongs in pull-requests.md as
it is information that is not specific to new Collaborators.) I am not
putting it in pullrequests.md because that document is already plenty
long and detailed, and includes advice on writing good commit messages,
which tend to become the pull request descriptions anyway.

PR-URL: https://github.com/nodejs/node/pull/21028
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

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