[avalanche] Do not bucket brigade (orphan) proof accessor

Summary:
The getProof() accessor can be removed so the peer manager is used directly. Incidentally the getOrphanProof() becomes free to remove as well.

Depends on D9768.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9769