rpc: Remove unused return type from appendCommand

Summary:
This is a backport of core#19717 [1/4]
https://github.com/bitcoin/bitcoin/pull/19717/commits/fa93bc14c7411a108dd024d391344fabf0f76369

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10145