fix: remove checked-in benchmark files

[mldangelo-oai]

Summary

• remove the checked-in benchmark workflow, benchmark suites, and benchmark report helper
• clean up workflow/script/docs references to the deleted benchmark lane
• keep the profiling utility and broader performance audit notes in place

Validation

• uv run ruff format modelaudit/ packages/modelaudit-picklescan/src packages/modelaudit-picklescan/tests tests/
• uv run ruff check --fix modelaudit/ packages/modelaudit-picklescan/src packages/modelaudit-picklescan/tests tests/
• uv run mypy modelaudit/ packages/modelaudit-picklescan/src packages/modelaudit-picklescan/tests tests/
• PROMPTFOO_DISABLE_TELEMETRY=1 uv run pytest -n auto -m "not slow and not integration" --maxfail=1
• npx prettier --check .github/workflows/README.md scripts/README.md docs/agents/performance-audit.md CHANGELOG.md