[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