Let testing/buildbot validate bot names against bot names in luci-milo.cfg

Remove all unreferenced bots this finds, and even a whole unused waterfall.

The testing/buildbot presubmit doesn't run on infra/config/global/luci-milo.cfg
changes, so it will instead fire on the first testing/buildbot change after
removing a bot, but that seems better than not having this check.

Bug: 843511
Change-Id: I8319329d01217cb5c4129c523f79ebd81732d8d9
Reviewed-on: https://chromium-review.googlesource.com/1061597
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559231}
9 files changed