Now that structs use a slice to store their fields, this code can be
simplified somewhat.

Passes toolstash -cmp.

Change-Id: If17b1c89871fa06f34938fa67df0f8c6bcf1a86b
Reviewed-on: https://go-review.googlesource.com/21219Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>