cmd/go: delete cmd/go/internal/slices in favor of slices

cmd/go: delete cmd/go/internal/slices in favor of slices

Some files already use "slices", others use "cmd/go/internal/slices".
(Some files are using more than slices.Clip and must use "slices".)
Use "slices" consistently and delete cmd/go/internal/slices.

Change-Id: I69ec680106ad2924276f7473e6547a3a907efc96
Reviewed-on: https://go-review.googlesource.com/c/go/+/499715
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>