Keep lint checks passing after backtest cleanup

The backtest script carried stale imports that made ruff fail even though runtime behavior did not depend on them. Removing the unused imports keeps CI lint checks green without changing script logic.

Confidence: high
Scope-risk: narrow
Reversibility: clean
Tested: python -m ruff check .
Not-tested: full application test suite

scripts/backtest_metar_calibrated_path.py

@@ -21,12 +21,11 @@ import argparse
 import csv
 import json
 import math
-import os
 import sqlite3
 import statistics
 import sys
 from dataclasses import dataclass
-from datetime import datetime, timedelta
+from datetime import datetime
 from pathlib import Path
 from typing import Any, Iterable