mgr/pg_autoscaler: drop undefined variable 'cr_name'

mgr/pg_autoscaler: drop undefined variable 'cr_name'

```
a/pg_autoscaler/module.py:359: error: Name 'cr_name' is not defined
```

Actually _get_pool_status should have prevented any pool
with a zero-ed capacity from going this far, so it is safe
to simply drop the obviously redundant check here.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>