Attempt at cleaning a couple things I missed in code review.
The old httpbin resource wasn't being used anyhow, so I just deduped
them and updated things in a couple other places that had drifted.
Hopefully this fixes the [flaky
CI](https://github.com/firezone/firezone/actions/runs/7918422653/job/21616835910)
This improves maintenance because we can now use a regular matrix for
the integration tests and one can locally use tools like shellcheck or a
`bash-lsp` during development.
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Signed-off-by: Jamil <jamilbk@users.noreply.github.com>
Co-authored-by: Jamil <jamilbk@users.noreply.github.com>