EDAC/wq: Remove unneeded flush_workqueue()

destroy_workqueue() already takes care of flushing the workqueue so
there is no need to flush it explicitly.

  [ bp: Massage commit message. ]

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: ran jianping <ran.jianping@zte.com.cn>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220424062127.3219542-1-ran.jianping@zte.com.cn