[swarming] enable cron jobs to trim TaskRequest and TaskOutputChunk
[swarming] enable cron jobs to trim TaskRequest and TaskOutputChunk

Respectively:
/internal/cron/delete_old_tasks
/internal/cron/delete_old_task_output_chunks

This is now safe as all instances had their versions prior 3880-ca6c194
were deleted.

Bug: 884579
Change-Id: I083dab9d84fb3ff550fe748a51dd54be38effbe0
Reviewed-on: https://chromium-review.googlesource.com/c/1351431
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