slices: Delete clears the tail when j == len(s)

slices: Delete clears the tail when j == len(s)

Fixes #65669

Change-Id: Ifd2011dd604fef399e4352b804fc2f6a9e74096e
Reviewed-on: https://go-review.googlesource.com/c/go/+/566237
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>