Remove the replace directive and update x/tools.

For golang/go#67936

Change-Id: I87b28c57607c2ded4c2e6b47ce725d4365151795
Reviewed-on: https://go-review.googlesource.com/c/tools/+/593677
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>