build: remove duplicate check for authors.yml

update-authors.js will print if there are duplicates in the new entries
so we can remove the (not working) duplication check in the authors.yml
file. It would still be nice to get the information about duplicates
into the body of the GitHub PR so no one has to manually check for
duplicates (by either examining the file or looking at the Actions
output), but that can be a future improvement.

PR-URL: https://github.com/nodejs/node/pull/40393


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>