How to remove group from PoolCluster?

Hello.

I need to remove some group of PoolCluster by pattern. For example:

```
poolCluster.add('shard1', config);
poolCluster.add('shard2', config);
poolCluster.add('shard3', config);
```

Sometime later I need to remove one of group:

```
poolCluster.remove('shard2');
```
