🐛(agents) fix shutdown exception in metadata extractor

Remove a redundant drain operation causing an exception during the
metadata extractor shutdown.

src/agents/metadata_collector.py

@@ -324,7 +324,6 @@ class MetadataCollector:
     async def _close_session(self, session: AgentSession) -> None:
         """Close and cleanup VAD monitoring session."""
         try:
-            await session.drain()
             await session.aclose()
         except Exception:
             logger.exception("Error closing session")