Remove client middleware from scope
Remove client middleware from scope

As per the discussion [1] on the FIG mailing list, attempting to define
a specification that work for client side middleware is ill advised.

It will be far better to address client middleware in a separate
proposal after the async proposal has been completed.

[1]: https://groups.google.com/d/msg/php-fig/vBk0BRgDe2s/GTaT0yKNBgAJ

http-interop/http-middleware