Cleanup CL https://golang.org/cl/41691 broke the plan9 build by removing
a use of a package but not removing the package import.

Trybots don't check that. I filed #20119 for that.

Change-Id: Ia030e6924665dfb871ca964455b899d51b0200c2
Reviewed-on: https://go-review.googlesource.com/41752Reviewed-by: David du Colombier <0intro@gmail.com>