chore: remove unused requirements.txt

[prabinoid]

What type of PR is this? (check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• 📝 Documentation
• 🧑‍💻 Refactor
• ✅ Test
• 🤖 Build or CI
• ❓ Other (please specify)

Related Issue

Part of #7183

Describe this PR

Removes the legacy requirements.txt from the repo root. Dependencies are
managed entirely by pyproject.toml + uv.lock, so the committed file was
stale and unused:

• The AWS CloudFormation deploy generates its own copy at runtime
  (uv export --no-dev > requirements.txt in
  scripts/aws/cloudformation/tasking-manager.template.js) — it never reads
  the committed one.
• The Docker build installs with uv sync --locked.

In the repo, the only references to requirements.txt
are the CloudFormation lines that create it. No CI workflow, Dockerfile, or
Makefile target consumes it.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud