[avalanche] Simplify contenders by treating rejection and invalidation the same

Summary: This provides a stronger guarantee that once a contender is finalized, it will never be reverted, even if the system managed to re-poll a finalized contender. We don't really care that invalidated contenders do not get removed from the winner set so long as blocks get mined with staking rewards going to a finalized contender.

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17710