all: make use of builtin clear

all: make use of builtin clear

Change-Id: I1df0685c75fc1044ba46003a69ecc7dfc53bbc2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/574675
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>