[🔥AUDIT🔥] Remove security issue template (#114)

🖍 _This is an audit!_ 🖍

## Summary:
I figured we should have this to say "STOP! Use HackerOne instead".
But GitHub anticipated that and already links to the security policy
(`docs/SECURITY.md`) as an option when [creating an issue][1], so
it's just duplicative.  In this commit I remove it.

[1]: https://github.com/Khan/genqlient/issues/new/choose

## Test plan:
above link should show just one option for security


Author: benjaminjkraft

Auditors: StevenACoffman

Required Reviewers: 

Approved By: 

Checks: ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Lint, ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Lint

Pull Request URL: #114

Author: benjaminjkraft