slices: optimize Delete
Makes Delete return early if no elements need to be deleted.
Change-Id: Id64f716b1529e9dd5972c920a54823dba75aafe9
GitHub-Last-Rev:
885c1afb5dcc76423791508e21ab84c67820efb2
GitHub-Pull-Request: golang/go#63411
Reviewed-on: https://go-review.googlesource.com/c/go/+/533276
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>