remove misleading comment

Summary:
-enableavalanche does not imply that the service flag is set (since D9422).
The code is simple enough to not require a comment.

Test Plan: NA

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9482