[avalanche] Remove the unused processor addNode() method

Summary:
The Peer Manager is used directly instead.

Depends on D9765 for the linter to be happy.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9766