github: remove workflow "Schutzbot comments"

This workflow was added with the plan to make it post a comment on a PR
when certain non-required checks failed, without failing the workflows
themselves.  However, this turns out to require a bit more work than
originally anticipated and is probably not worth the effort right now.

It might be worth waiting for github to add support for something like
this, or we could make a github action for ourselves that abstracts this
idea to make it reusable across projects.

For now, I'd like to remove the workflow so it doesn't stay in the repo
unfinished as clutter.

This reverts commit 0fc99d2.