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>

wrap.go

diff --git a/wrap.go b/wrap.go
index 50de474..9a3b510 100644
--- a/wrap.go
+++ b/wrap.go
@@ -52,7 +52,7 @@
 		return err == target
 	}
 
-	isComparable := target == nil || reflect.TypeOf(target).Comparable()
+	isComparable := reflect.TypeOf(target).Comparable()
 	for {
 		if isComparable && err == target {
 			return true