Remove FUNDING.yml

We have instead moved thoughtbot's global `FUNDING.yml` to the
[`thoughtbot/.github`][.github-repo] repo. It's a special repo that
GitHub supports for [setting default community health
files][github-docs].

[.github-repo]: https://github.com/thoughtbot/.github
[github-docs]: https://help.github.com/en/github/building-a-strong-community/creating-a-default-community-health-file-for-your-organization