fix: resolve ruff lint errors in benchmark suite

Remove unused imports (shutil, string, datetime, os, yaml, time,
SCALE_CONFIGS) and unused variable assignments in timing-only calls.

tests/benchmarks/test_recall_threshold.py

@@ -8,7 +8,6 @@ wing+room to find the actual embedding model limit.
 
 import hashlib
 import os
-import time
 from datetime import datetime
 
 import chromadb