Remove unnecessary 'flatten'

The `alt_urls` method [already calls `flatten`](https://github.com/jekyll/jekyll-redirect-from/blob/a47186219f8f64dd17c27bc2fb264ab4667d0fe9/lib/jekyll-redirect-from/redirector.rb#L39) on its array. Presumably we don't need to call `flatten` again on that return value?

/cc @gjtorikian @parkr