Fixed bug in pool_rm

When removing a pool it was not possible to create a new one with the same name as in:

```erlang
Spec = [{name, pool_a}...],
pooler:new_pool(Spec), % => {ok, <some.pid.yap>}
pooler:rm_pool(pool_a}...]),
pooler:new_pool(Spec), % => {error, already_present}
```
the original code for rm_pool read:

```erlang
rm_pool(Name) ->
    SupName = pool_sup_name(Name),
    case supervisor:terminate_child(?MODULE, SupName) of
        {error, not_found} ->
            ok;
        ok ->
            supervisor:terminate_child(?MODULE, SupName);
        Error ->
            Error
    end.
```

Line 7 calls `supervisor:terminate_child` a second time, my guess is that actually `supervisor:delete_child` was meant here to remove the child from the supervisor so it can be added again later.