fix includes in standard.{h|cpp}

Summary:
As far as I can tell, nothing in this module needs amount.h, so just remove the include. This should prevent D15778 from introducing a new amount -> util/system -> script/standard -> amount circular import.

While we are fixing the includes, remove also the unused crypto/sha256.h include in the source file.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15780