Disabling ignoring unused vars for _ prefix

Created by: gaearon

We currently ignore unused vars with _ prefix, mainly as a workaround for deleting extra properties in ...rest destructuring. However https://github.com/facebookincubator/create-react-app/pull/1705 is a proper fix for this, so I think we should remove the _ rule in the next major bump. This would fix https://github.com/facebookincubator/create-react-app/issues/1387.