doc: remove subsystem from pull request template

Remove request that user provide a list of subsystems in the pull
request template.

* We already ask them to put the subsystems in the first line of the
  commit message.
* The subsystem is usually easy to determine.
* We have a bot that applies subsystem labels on new PRs and it seems to
  do a rather good job. Tools over rules. Let the bot do it.
* The fewer unnecessary things we ask for in the template, the lower the
  barrier to entry.
* Anecdotal, but I have never found it useful to have the person
  submitting the PR list out subsystems in the pull request post.

PR-URL: #19125
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matheus Marchini <matheus@sthima.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Kyle Farnung <kfarnung@microsoft.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -14,6 +14,3 @@ Contributors guide: https://github.com/nodejs/node/blob/master/CONTRIBUTING.md
 - [ ] tests and/or benchmarks are included
 - [ ] documentation is changed or added
 - [ ] commit message follows [commit guidelines](https://github.com/nodejs/node/blob/master/doc/guides/contributing/pull-requests.md#commit-message-guidelines)
-
-##### Affected core subsystem(s)
-<!-- Provide affected core subsystem(s) (like doc, cluster, crypto, etc). -->