hack: use "latest" apidiff

Pinning apidiff to a specific version shouldn't be necessary because, if past
experience in klog holds true, the latest version just works. This way we don't
have to remember to bump up the revision.

The downside of using "latest" is that a compromise of that version would give
an attacker the ability to run code in the CI and on developer machines.

hack/tools/tools.go

@@ -36,5 +36,4 @@ import (
 
 	// tools like cpu
 	_ "go.uber.org/automaxprocs"
-	_ "golang.org/x/exp/cmd/apidiff"
 )