all: use slices.Contains to simplify code

all: use slices.Contains to simplify code

Change-Id: I9ef075bbb0e3c65f3c2a9d49e599ef50b18aa9be
Reviewed-on: https://go-review.googlesource.com/c/go/+/639535
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>