xerrors: remove useless condition checking in Is Analogous change to golang.org/cl/176497 Change-Id: I9296af11c53f1573f550f88ec14b55ed6ded3128 Reviewed-on: https://go-review.googlesource.com/c/xerrors/+/176517 Reviewed-by: Andrew Bonventre <andybons@golang.org>