Remove `Hanami::Action#parsed_request_body`

[Luca Guidi]

Hanami::Action#parsed_request_body is deprecated in 1.3 and should be removed in 2.0.

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This change MUST go to the unstable branch.

NOTE: Due to the changes that unstable branch contains Hanami::Action#parsed_request_body is referenced as Hanami::Action::Request#parsed_body. This is the method to be removed.

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Ref #254
Ref hanami/router#163

[Gustavo Caso]

Closed by #261