[avalanche] Consistently use the default constant when checking for preconsensus

Summary: As more code will use this option flag, make sure we can turn it all at the same time with a single line change.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15742