Remove unused char serialize
Summary:
This concludes backport of core#23438
https://github.com/bitcoin/bitcoin/pull/23438/commits/fa5d2e678c809c26bd40d7e7c171529d3ffb5903
Depends on D15935
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D15936