[avalanche] Cleanup unused addStakeContender in Processor

Summary: Adding contenders to the cache is entirely handled within PeerManager now, so this is unused and can be cleaned up.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17880