ci: remove unused import causing pre_check to fail [backport 2.1] (#7651)

Backport 14c7d01 from #7636 to 2.1.

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
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[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
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implications of this PR as reported in the benchmarks PR comment.
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[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)
- [x] If this PR touches code that signs or publishes builds or
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from `@DataDog/security-design-and-guidance`.
- [x] This PR doesn't touch any of that.

Co-authored-by: Emmett Butler <[email protected]>

Author: github-actions[bot]

tests/contrib/flask/test_request.py

@@ -1,7 +1,6 @@
 # -*- coding: utf-8 -*-
 import json
 import os
-import signal
 import subprocess
 import sys
 import time