[avalanche] Don't request orphaned proofs

Summary:
If a proof is orphaned we know about it already.

Ref T1611.

Test Plan:

./test/functional/test_runner.py p2p_inv_download

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: deadalnix

Maniphest Tasks: T1611

Differential Revision: https://reviews.bitcoinabc.org/D9660