remove 'rawBody' request-attribute

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As @weierophinney mention in another https://github.com/zendframework/zend-expressive-helpers/pull/64#discussion_r204831760, we don't really need the rawBody attribute on the request, as the body itself can always be retrieved via Request::getBody()

[weierophinney]

This repository has been closed and moved to mezzio/mezzio-helpers; a new issue has been opened at https://github.com/mezzio/mezzio-helpers/issues/2.