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- Remove support for 'key_removals', which is unused after
  https://chromium-review.googlesource.com/c/1136516/
-- As a knock-on effect, update_and_cleanup_test() can now no longer
   throw, so remove several try:/except: blocks to keep the coverage
   script happy.
- Remove support for having "$bot_name $waterfall_name" in
  'modifications' since that's no longer needed and there was a TODO
  about removing it already.

Bug: 843511
Change-Id: I2d9d89e60ad404f78c97f448db017091b4d18b57
Reviewed-on: https://chromium-review.googlesource.com/1136893
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575022}