[avalanche] Simplify the processor test item providers by removing registerVotes specialization

Summary:
Now that we have a variant VoteItem, there is no need anymore for the specialized methods in the providers.
There is no change in behavior.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13061