Cleanup reject_code in p2p_invalid_block.py

Summary:
Partial backport of Core PR14119
https://github.com/bitcoin/bitcoin/pull/14119/files#diff-bb164712e06aa669cbef899af79e8cdb

Test Plan: test_runner.py p2p_invalid_block

Reviewers: #bitcoin_abc, deadalnix, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D3874