[swarming] remove TODO in signal_trace_test.py
[swarming] remove TODO in signal_trace_test.py

signal_trace.py is for platforms other than windows, so no need to be tested on windows.
Therefore remove the TODO.

Bug: 1017545
Change-Id: I805a64ffebbe5b02cce51e73786556c5603c4eac
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2830282
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Momo Sasaki <e99fedc53ffce896a4ecf43ecee40bbdb01d5909@google.com>

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