[avalanche] Clear the remote proofs upon node removal

Summary:
If a peer disconnects, clear its remote proofs set.

Depends on D14659.

Test Plan:

ninja check-avalanche-peermanager_tests

Reviewers: #bitcoin_abc, PiRK, bytesofman

Reviewed By: #bitcoin_abc, PiRK, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14660