image: use built-in clear to simplify code
Change-Id: Id34936a115baaf61e4268582c6d9a2027494c385
GitHub-Last-Rev:
5fe455b7d24e3e3b871c8999c5bb534f3e1e3ab5
GitHub-Pull-Request: golang/go#66244
Reviewed-on: https://go-review.googlesource.com/c/go/+/570555
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Keith Randall <khr@golang.org>