[avalanche] Don't expire finalized transactions

Summary:
These txs should never turn invalid. As a consequence they should never be removed from the mempool/finalized tree after they are finalized.

Depends on D18233.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_transaction_finalization

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18234