[swarming] do not extend deadline in _cancel_task_tx()
[swarming] do not extend deadline in _cancel_task_tx()

This seems to cause a delay killing tasks.

Bug: 1183873
Change-Id: I70e53042f3fac29f63ad6e8761ad844519e9ef57
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2777482
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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