TASK: Remove Flow's RequestInterface

TLTR:

Since the separation of http and cli requests the interface is completely unused. And strict types against its only implementation, the ActionRequest make it impossible to implement a custom request.

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_Initially_ the interface was introduced with 2a26925 at beginning of time. At that time this was done probably to be able to type against the interface instead of the common `Request` class, that the Cli and Web Request both extended.

With the removal of many methods in its interface 8c40ff2 and with the full separation of action and cli requests #1552 the interface became useless.

The dispatcher and controller interface are both typed against the `ActionRequest` and so it's impossible to implement a custom request.

Neos.Flow/Classes/Mvc/ActionRequest.php

@@ -26,7 +26,7 @@
  *
  * @api
  */
-class ActionRequest implements RequestInterface
+class ActionRequest
 {
     /**
      * @Flow\Inject