Cleanup reject_code in abc-schnorr.py

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

Test Plan: test_runner.py abc-schnorr

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3923