Remove not-belongs-to as not belongs-to is equivalent

Added in #220. I don't think this is necessary as `not-belongs-to` can be simply called as `not belongs-to` [like here](https://github.com/thoughtbot/neat/blob/master/app/assets/stylesheets/functions/_new-breakpoint.scss#L44). Whereas in `belongs-to` it's now clear that you're calling `not not` in front of index. As sass community picks up `not not` as JS `!!` this make the code more self explanatory.

/cc @kaishin.