clean up CheckInputs() policy-ban logic and comments

Summary:
I think this variable change makes it clearer what is actually going on.

Also, the comments here were out-of-date, referring to the now-mandatory flag STRICTENC, and referring to monolith upgrade.

Test Plan: make check and run functional tests

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2511