doc: clarify expectations for PR commit messages

Prior to this commit, new contributors were suggested
to use a utility to validate commit messages. Although
not inaccurate, this utility produces misleading results.

* Remove reference to `core-validate-commit`

PR-URL: #30922
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/guides/contributing/pull-requests.md

@@ -192,9 +192,6 @@ One of the existing contributors will help get things situated and the
 contributor landing the Pull Request will ensure that everything follows
 the project guidelines.
 
-See [core-validate-commit](https://github.com/nodejs/core-validate-commit) -
-A utility that ensures commits follow the commit formatting guidelines.
-
 ### Step 5: Rebase
 
 As a best practice, once you have committed your changes, it is a good idea