Remove the test workaround for IMPALA-1047.

I ran 330 iterations of the test (~13 hours) without a single failure.
Also inspected the Cancel code and didn't notice any potential races
that could lead to this.  If anyone knows some secret recipe for
reproducing this, please let me know.  Otherwise, let's remove the
workaround in the test.

Change-Id: I216ed6416e150a790b8453afe5890efadf472739
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3720
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins