Remove unused asyncio import from usage.py

Removed unused asyncio import since we switched to threading.Lock
instead of asyncio.Lock for synchronous thread-safe operations.

Author: certainly-param

pydantic_ai_slim/pydantic_ai/usage.py

@@ -1,6 +1,5 @@
 from __future__ import annotations as _annotations
 
-import asyncio
 import dataclasses
 import threading
 from copy import copy