[swarming] enable cron job to delete stale bot
[swarming] enable cron job to delete stale bot

Bug: 884579
Change-Id: I009f1f755de80996e00b5f7605d75eee3420867a
Reviewed-on: https://chromium-review.googlesource.com/c/1365353
Reviewed-by: Quinten Yearsley <747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py