Cleanup reject_code in abc-schnorrmultisig-activation

Summary:
Cleanup related to backporting Core PR14119
https://github.com/bitcoin/bitcoin/pull/14119/files

Depends on D3862

Test Plan: test_runner.py abc-schnorrmultisig-activation

Reviewers: #bitcoin_abc, deadalnix, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3895