tools/testing/nvdimm: remove unneeded flush_workqueue

All work currently pending will be done first by calling destroy_workqueue,
so there is no need to flush it explicitly.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: ran jianping <ran.jianping@zte.com.cn>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20220424062655.3221152-1-ran.jianping@zte.com.cn


Signed-off-by: Dan Williams <dan.j.williams@intel.com>