group link cache clean up logic looks wrong

https://github.com/HDFGroup/h5pyd/blob/master/h5pyd/_hl/group.py#L978
```
for n in name:
if n.find('/') == -1 and n in self._link_db:
# remove from link cache
del self._link_db[name]
```

If `name` is a list, it would attempt to delete the list which doesn't exist in the _link_db cache.
If `name` is a string, it would iterate the characters so `n` wouldn't be in the _link_db cache.