[avalanche] Simplify and deduplicate tx finalization log

Summary: The tx finalization log currently happens in 2 places which is not the best and also causes a subtle log duplication in the event a tx is being finalized and was also the parent of another tx finalized in the voting same round. This simplified design makes sure it can't happen anymore.

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18501