doc: remove breaking-change-helper from onboarding-extras

I don't think anyone uses the breaking change helper code that is in
onboarding-extras. I'm not sure it works for us anyway as I think it
will fail if there are any breaking changes since the last release. That
may be helpful to releasers, but that is not helpful in determining if
an individual commit is semver-major or not. Regardless, I don't think
it gets used and I'm interested in not weighing down the onboarding
process with a bunch of things that we don't actually use/do.

PR-URL: https://github.com/nodejs/node/pull/34497


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Pranshu Srivastava <rexagod@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>