[avalanche] Remove finalized txs upon block finalization

Summary:
Make sure the txs remain finalized after a block is mined.
Note that we still need to remove the txs from the radix tree if another
event (not a block) is causing the mempool removal.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16019