[avalanche] Cleanup unnecessary API passthrough of getWinners

Summary:
Consolidate the logic for determining the new stake winners in peerManager. This makes it more clear that winners should be updated every time a contender is finalized via avalanche and eliminates the API passthrough to getWinners, which is unnecessary.

Depends on D17881

Test Plan:

ninja && ninja check && ./test/functional/test_runner.py abc_p2p_avalanche_contender_voting

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17882