[milo] Remove unused fields from BuildSummary.
[milo] Remove unused fields from BuildSummary.

R=2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org, nodir@chromium.org

Bug: 758007
Change-Id: I3354da57f5c81c04a0533dc5c3afe4c88a26709b
Reviewed-on: https://chromium-review.googlesource.com/727475
Reviewed-by: Ryan Tseng <2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org>
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go