Remove ::IsStandardTx(tx, reason) alias

Summary:
Apart from tests, it is only used in one place, so there is no need for
an alias.

This is a partial backport of core#25648
https://github.com/bitcoin/bitcoin/pull/25648/commits/fa8a7f01fe1b6db98097021276ed5d929faadbec

Depends on D15754

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15755