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`
when working on commit b874747b51882a613895a100c4210c7f1dddde30 of PR15639,
that looks wrong in hindsight
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