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: #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>

doc/guides/onboarding-extras.md

@@ -46,14 +46,6 @@ such as if CI run fails or a new outstanding review comment is posted.
     section to the docs"
   * major vs. everything else: run last versions tests against this version, if
     they pass, **probably** minor or patch
-  * A breaking change helper
-    ([full source](https://gist.github.com/chrisdickinson/ba532fa0e4e243fb7b44)):
-
-    ```bash
-    SHOW=$(git show-ref -d $(git describe --abbrev=0) | tail -n1 | awk '{print $1}')
-    git checkout $(git show -s --pretty='%T' $SHOW) -- test
-    make -j4 test
-    ```
 
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