[avalanche] Fix an log error during proof rejection

Summary: The message ERROR: Failed to reject proof is printed when a proof is rejected due to conflicts. This diff fixes the issue by simply using the output from the PeerManager::rejectProof() method instead of another layer of logic.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_proof_voting

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11756