[avalanche] Cleanup redundant proof count getters in PeerManager API

Summary:
The pools can be accessed directly by the caller so there is no need for PeerManager to provide this API anymore.

Depends on D12062

Test Plan:

ninja
./test/functional/test_runner.py abc_rpc_getavalancheinfo
ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12064