[avalanche] Don't reserve a slot for block voting

Summary:
This was intended to be a DoS mitigation but is no longer required.

Ref T1854.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10365