meta: simplify mailmap

Remove unnecessary name specificity in mailmap entries. AUTHORS is
unchanged after running update-authors.js with these changes.

PR-URL: https://github.com/nodejs/node/pull/39612


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>