Cleanup reject_code in feature_block.py

Summary:
Partial backport of Core PR14119
https://github.com/bitcoin/bitcoin/pull/14119/files#diff-3c4697a632e54ae2c6e68e0159ec2234

Depends on D3862

Test Plan: test_runner.py feature_block

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3873