Remove ::fIsBareMultisigStd global

Summary:
This is a backport of core#25648
https://github.com/bitcoin/bitcoin/pull/25648/commits/fa2f6c1a611dffe5a3f63fe1b453f1dd420371b1

Depends on D15755

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15757