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: #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>

.github/PULL_REQUEST_TEMPLATE.md

@@ -7,10 +7,6 @@ Bug fixes and new features should include tests and possibly benchmarks.
 Contributors guide: https://github.com/nodejs/node/blob/master/CONTRIBUTING.md
 -->
 
-#### Related Issues
-
-Fixes: https://github.com/nodejs/node/issues/<issue_number>
-
 ##### Checklist
 <!-- Remove items that do not apply. For completed items, change [ ] to [x]. -->