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 <[email protected]>
Auto-Submit: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>