[avalanche] Slightly reduce cs_main scope in registerVotes

Summary:
This is a follow-up to D10379. Since IsWorthPolling() also requires the lock, the WITH_LOCK() macro is not used here. There is no change in behavior.

Ref T1854.

Depends on D10379.

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10391