all: use slices.Delete

all: use slices.Delete

Change-Id: Ifb6aa07b32127907cdc2df44b2dbddd6296775c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/616737
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>