[avalanche] Inline getPeerScoreFromNodeId

Summary:
There is a single callsite outside of the test, just inline it.
This is a first step of refactor for the remote proofs mechanism.

Test Plan:

ninja check-avalanche-peermanager_tests

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14956