Drop JSONRPCRequest constructors

Summary:
This just makes an additional simplification after D11552 replaced util::Ref with std::any.

Note that we sometimes wrap the context in a HTTPRPCRequestProcessorinstance, so some changes don't apply in bitcoind.cpp and init.cpp.

This is a backport of core#21574
Depends on D11552

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11560