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: #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>

doc/onboarding.md

@@ -185,9 +185,7 @@ onboarding session.
       request containing the code you wish to test. For example, if the URL for
       the pull request is `https://github.com/nodejs/node/issues/7006`, then put
       `7006` in the `PR_ID`.
-    * The remaining elements on the form are typically unchanged with the
-      exception of `POST_STATUS_TO_PR`. Check that if you want a CI status
-      indicator to be automatically inserted into the PR.
+    * The remaining elements on the form are typically unchanged.
   * If you need help with something CI-related:
     * Use #node-dev (IRC) to talk to other Collaborators.
     * Use #node-build (IRC) to talk to the Build WG members who maintain the CI