The value of `auto_size` is not judged when the worker is removed

[tigercl]

When you do not use auto_size, every time you remove a worker, the size of the pool will become smaller. When it becomes 0, you will never be able to add a worker.

[tigercl]

I think the problem is in function del_slot, the following part of the code should be redundant:

del_slot(Name, [{Name,_}]) ->
    [];

At the same time, the code in do_remove_worker_ seems to be missing the processing of auto_size.