Remove the golang.org/x/exp requirement once unused

Drop the direct require golang.org/x/exp directive from go.mod once no source file needs it — either because nothing imports it any more, or because the migration moves every import that remains. x/exp holds far more than the packages these recipes cover, so an import of any other one keeps the requirement, as does an // indirect entry. Does not touch go.sum, so a go mod tidy is still needed.