doc: remove "Related Issues" from pull request template

We don't require an issue for a pull request, so this section usually
ends up displaying confusing default content.

PR-URL: https://github.com/nodejs/node/pull/36590


Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>