[swarming] extract run function from cancel_task
[swarming] extract run function from cancel_task

I will call _cancel_task_tx inside _bot_update_tx.

Bug: 878895
Change-Id: I7cfc30851e40c232980a8aad281a9ede00b60e9e
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1687629
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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