Allows removing a bunch of unnecessary fields.

Passes toolstash/buildall.

Change-Id: Iec2492920e1c3ef352a9bf4296c74a55d9cc9ad6
Reviewed-on: https://go-review.googlesource.com/20677Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>