Delete .github/workflows/semgrep.yml (#391)

## What was changed
Delete the Semgrep workflow file in this repo.

## Why?
Now that the Semgrep CI workflow is shared and runs on every repo, this
per-repo file is no longer needed.

## Checklist

### How was this tested:

Saw the org-level action show up in the Actions tab in other repos and
it runs successfully.

Author: jackdawm