delete more worker than expected when to downclock pool

[hawkingrei]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

delete more worker than expected when to downclock pool.

First, you need to overclock the pool many times. then to downclock, it will happen this bug.

2. What did you expect to see? (Required)

delete one worker

3. What did you see instead (Required)

delete multi worker

4. What is your TiDB version? (Required)