cmd: use built-in clear for maps instead of range+delete

cmd: use built-in clear for maps instead of range+delete

Now that we're bootstrapping from a toolchain that has the clear builtin.

Update #64751

Change-Id: Ia86d96c253c9f7c66131cd02048a493047569641
Reviewed-on: https://go-review.googlesource.com/c/go/+/610237
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>