Cleanup reject_code in feature_csv_activation.py

Summary:
Partial backport of Core PR14119
https://github.com/bitcoin/bitcoin/pull/14119/files#diff-72d4bdc0bac8c2153666b0a94544af94

Test Plan: test_runner.py feature_csv_activation

Reviewers: #bitcoin_abc, deadalnix, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D3877