doc: remove POST_STATUS_TO_PR from onboarding.md

POST_STATUS_TO_PR is checked/enabled by default so there is no longer a
need to mention that it should be checked in onboarding.md. There is
almost never a situation when it should be unchecked.

PR-URL: https://github.com/nodejs/node/pull/21042


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>