[avalanche] Remove requirement that local staking rewards have to be computed in order to poll contenders

Summary: This is more robust in the event that staking rewards fails to compute for a minority of nodes. We had this happen on one node during testnet testing.

Test Plan:

ninja check
./test/functional/test_runner.py abc_p2p_avalanche_contender_voting

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17917