[avalanche] Add a debug check that we never remove a finalized transaction for conflict

Summary: We reject blocks that contain a tx that conflicts with a finalized one, so this assumption should never break. It's cheap enough that we can assert this on debug bulds to increase confidence.

Test Plan:
With debug:

ninja check-functional

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18209