Build fix for plan9 after 8bf13838.

Change-Id: I66d923abbef13ba35c76c6f6da5b14c6d534127c
Reviewed-on: https://go-review.googlesource.com/4165Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: David du Colombier <0intro@gmail.com>

src/time/zoneinfo_plan9.go

@@ -7,7 +7,6 @@
 package time
 
 import (
-	"errors"
 	"runtime"
 	"syscall"
 )