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>
1 file changed