chore: remove issue template in prep for open-sourcing (#28)

This repo's issue template was carried over from an internal repo. It
contains placeholder values for GitHub handles that are intended to be
replaced by real ones, but those handles already are real users here on
GitHub.com, so we'd be spamming them with each new issues if we were to
keep this in.

In addition, this template isn't used in any of our other soon-to-be
open-sourced repos, so it feels like an oddball. This PR removes it
altogether for now. We may re-add a template in the future, but it will
be consistent with other repos.