[avalanche] replace banscore int with disconnect bool in registerVotes

Summary: The banscore was always 0 or 100 (which causes disconnection), so we can simplify the logic by using a boolean without changing behavior.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18016