os: handle 'has been removed' error as IsNotExist on Plan 9

It fixes the TestRemoveAllRace test introduced
in CL 145860043.

LGTM=bradfitz
R=rsc, bradfitz
CC=golang-codereviews
https://golang.org/cl/147820044

src/os/error_plan9.go

@@ -25,7 +25,7 @@ func isNotExist(err error) bool {
 	case *LinkError:
 		err = pe.Err
 	}
-	return contains(err.Error(), "does not exist") || contains(err.Error(), "not found")
+	return contains(err.Error(), "does not exist") || contains(err.Error(), "not found") || contains(err.Error(), "has been removed")
 }
 
 func isPermission(err error) bool {