[avalanche] Deprecate the peer availability score

Summary:
This score was added in D13073 and D13424 but was never used. It's causing more confusion than it is helpful so better remove it.

Depends on D17668.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D17669