user32/tests: Remove variable atom which is not really used from test_Expose.

[Gerald Pfeifer]

On Sun, 9 May 2010, testbot at testbot.winehq.org wrote:
> Full results can be found at
> http://testbot.winehq.org/JobDetails.pl?Key=1983

Another false positive.

  W7PRO                completed 39
  W7PROX64             completed 19
  W7PROX64             completed 19

...is the same before and after this patch of mine.

Gerald