[avalanche] Simplify the proof verification code in the peer manager

Summary:
As per review https://reviews.bitcoinabc.org/D10522#inline-49160, there is no point passing the state as a parameter if it is already captured. This makes it possible to simplify the code a bit.

Ref T1854.

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10556