Deadlines aren't implemented on Plan 9 yet.

Updates #17477.

Change-Id: I44ffdbef97276dfec56547e5189672b7da24bfc1
Reviewed-on: https://go-review.googlesource.com/31188Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>