Remove .order file since it is not needed.

I can't see practical use of `.order` file. 

It was used by @pietern in `order` [rake task](https://github.com/redis/redis-rb/commit/7b85262efa9fe6251ed1daad1003cd88efd61e62), but this task was remove from `Rakefile` in [this commit](https://github.com/redis/redis-rb/commit/d9dd99919aae596ee71d611a6bc4162218e72dda) by @djanowski. 

So I guess `.order` file can also go away.