rpc: [refactor] Use concise C++11 code in CRPCConvertTable constructor
Summary:
This is a backport of core#20012 [1/3]
https://github.com/bitcoin/bitcoin/pull/20012/commits/faf835680be39811827504f77005b6603165f53e
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D10759