[avalanche] Kill PeerManager::addPeer

Summary:
This removes part of the Peer based API. It is replaces by a Proof based API: getPeer.

Depends on D6850

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6851