touchscreen_calibration: fix unused_event name

dummy_ variables were deprecated and must be replaced by unused_
variables.

BUG=none
TEST=make lint # no more W0613 in pytests.

Change-Id: I3489a6ea541a1266d21e319ea4e59a11937b6916
Reviewed-on: https://chromium-review.googlesource.com/202952
Reviewed-by: Ricky Liang <jcliang@chromium.org>
Commit-Queue: Shyh-In Hwang <josephsih@chromium.org>
Tested-by: Shyh-In Hwang <josephsih@chromium.org>
1 file changed