RequestHandler::deserializeBody() passes 'request_method' to as deserialization context, never used

Introduced in 54feb2f6eeb21bfefdaa0c61e45d629f12c1c5dd, but never used for anything. Safe to delete.

As part of #2829398: Clean up JsonApiResource: the annotation, plugin type, plugin manager, plugin implementations, the dynamic routes generator and the request handler, I'm working on simplifying the serialization context, because it effectively amounts to global state. If we have any serialization context, we must be able to explain its use, and this case, I found there was zero use altogether, so we can just delete it! :)