[avalanche] Fix a race in abc_p2p_avalanche_contender_voting

Summary: Looking at the log while checking the isfinalrpc is racy and not redundant.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_contender_voting.py

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17284