cmd: use cmd/internal/hash.New32 and Sum32 only

cmd: use cmd/internal/hash.New32 and Sum32 only

Do not use New16, New20, Sum16, Sum20 anymore.
As of CL 641096, these are just wrappers around New32 and Sum32.
Change call sites to use them directly.

Change-Id: Icea91a77449f6839b903894997057ba404bd04e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/641076
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>