Fix tst_combobox

There was a bogus timer firing once after the test had been running 500 ms. The
timer was supposed to be used as a workaround on Mac, but since it had running
set to true, it was actually firing once on all platforms.

The timer was originally added in 6254af55

. The test function ended up being
skipped on Mac, so the timer was actually pointless.

Task-number: QTBUG-64664
Reviewed-by: Liang Qi <liang.qi@qt.io>
(cherry picked from commit a7495251

)
Change-Id: Icebda05c872ba2de690e87bd0a9968670f203ce2
Reviewed-by: J-P Nurmi <jpnurmi@qt.io>