fix wrong include prior to backporting 15639

Summary:
in D5967 (core commit) I've included policy/settings.h in policy/policy.h
so that our version of GetVirtualTransactionSize still had access to
::nBytesPerSigOp

recently while working on commit b874747b51882a613895a100c4210c7f1dddde30 of PR15639,
I realize, in hindsight, that looks wrong

this diff reverts the include I added to policy.h, moves our version of
GetVirtualTransactionSize to settings.h ahead of PR15639 and makes
the other necessary changes

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6253