Remove the replace directive and update x/tools.

For golang/go#66730

Change-Id: I012605bf892ba8ca86b9c068832117c68fb68928
Reviewed-on: https://go-review.googlesource.com/c/tools/+/578321
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>