cmd: remove notsha256 package

cmd: remove notsha256 package

All of its usages were removed in CL 610596.

Updates #51940
Updates #64751

Change-Id: I72a0ea4bd44a2f671e032bffa1facf84822a90f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/610599
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>