[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