[avalanche] Don't assume the legacy proof format in get_proof_ids

Summary: This is a step toward migrating our tests to the new proof format. This removes one of the assumption that legacy format is used.

Test Plan:

ninja check-extended

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11117