Revert "run_isolated: fallback to python if go failed"
Revert "run_isolated: fallback to python if go failed"

This reverts commit 6a5be7ed0deee29cdc606b27fff9d247a87cfefa.

Reason for revert:
Fallback path is added in crrev.com/c/1960062
But this increases implementation complexity.

So let me remove fallback path, if we see the same error, I will
increase retry count introduced in crrev.com/c/1958395

Original change's description:
> run_isolated: fallback to python if go failed
> 
> This is for failure like
> https://chromium-swarm.appspot.com/task?id=490c7d1721223710
> 
> Bug: 932396
> Change-Id: Iee02df95f83a122a1493815f414bb0e3b0a18794
> Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1960062
> Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
> Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
> Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

TBR=maruel@chromium.org,71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org,aaef61ef98ffea505db831f310be2a0eb8492537@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 932396
Change-Id: I0433820d481da6dec340d873bdfcd2da0be07f81
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1990927
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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