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Remove chromium.clang "tester" bots

These bots are no longer split between builders and testers.

Part 2 after https://chromium-review.googlesource.com/783518

R=hans@chromium.org

No-Try: true
Bug: 587879
Change-Id: I98201d6582933e1e86e3762abfe2923bdad7c21e
Reviewed-on: https://chromium-review.googlesource.com/792346
Commit-Queue: Reid Kleckner <rnk@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519748}