Do not override `extractMeta`

I think the default implementation of `extractMeta` in `RESTSerializer` is fine for JSON API: https://github.com/emberjs/data/blob/v1.0.0-beta.12/packages/ember-data/lib/serializers/json_serializer.js#L953

```js
  extractMeta: function(store, type, payload) {
    if (payload && payload.meta) {
      store.metaForType(type, payload.meta);
      delete payload.meta;
    }
  }
```