rpc: drop unused JSONRPCProcessBatchReply size arg, refactor

Summary:
This is a backport of Core PR18594 [5/6]
https://github.com/bitcoin/bitcoin/pull/18594/commits/903b6c117f541ea9258d3234ffcf59427344e668

Depends on D9240

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9241