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.

Backport-PR-URL: https://github.com/nodejs/node/pull/22380
PR-URL: https://github.com/nodejs/node/pull/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: Сковород...