[avalanche] Cleanup unnecessary setArgs in compute_staking_rewards test

Summary: These args are already set to 0 in the fixture. I could have set these to be useful values, but that test coverage is already handled in other tests so it is unnecessarily verbose.

Test Plan:

ninja check-avalanche-processor_tests

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17160