unset key from local storage

Currently, I deal with local storage like that:

```
let mut local_storage_token = use_synced_storage::>("x_auth_token".to_string(), || None);

*local_storage_token.write() = None;
```

As a result, the key exists in local storage (value `None`).

It would be great to have some alternative for `*local_storage_token.write() = None;`