errors: remove useless condition checking in Is

golang.org/cl/175260 fixed Is panics if target is uncomparable. It did
add an useless condition checking whether target is comparable. Just
remove that condition.

Change-Id: I0a317056479638d209b0a0cbc7010c153558c087
Reviewed-on: https://go-review.googlesource.com/c/go/+/176497Reviewed-by: Joan Lopez de la Franca Beltran <joanjan14@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/errors/wrap.go

@@ -52,7 +52,7 @@ func Is(err, target error) bool {
 		return err == target
 	}
 
-	isComparable := target == nil || reflectlite.TypeOf(target).Comparable()
+	isComparable := reflectlite.TypeOf(target).Comparable()
 	for {
 		if isComparable && err == target {
 			return true