[skip ci] Remove unused package of locust_task (#11878)

Signed-off-by: Nwangting0128 <ting.wang@zilliz.com>

tests/benchmark/milvus_benchmark/runners/locust_task.py

 import time
-import pdb
 import logging
 from locust import User, events
 from milvus_benchmark.client import MilvusClient