Drop 'help or discussion' issue template

Per https://github.com/serde-rs/serde#getting-help we're steering these
to Discord / Stack Overflow / Reddit / etc.

Can reconsider when GitHub Discussions is out of beta
(https://github.com/vercel/next.js/discussions).

.github/ISSUE_TEMPLATE/4-discussion.md

@@ -1,7 +0,0 @@
----
-name: Help or discussion
-about: This is the right place
-
----
-
-