slices,sort: explicitly discard results in benchmarks

slices,sort: explicitly discard results in benchmarks

The unusedresult analyzer will report failure to use the results
of these pure functions.

Updates #73950

Change-Id: I783cb92ad913105afd46c782bedf6234410c645d
Reviewed-on: https://go-review.googlesource.com/c/go/+/677995
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Commit-Queue: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>