memory_hotplug: drop spurious calls to flush_scheduled_work()

lru_add_drain_all() uses schedule_on_each_cpu() which is synchronous.
There is no reason to call flush_scheduled_work() after
lru_add_drain_all().  Drop the spurious calls.

This is to prepare for the deprecation and removal of
flush_scheduled_work().

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>

mm/memory_hotplug.c

@@ -840,7 +840,6 @@ static int offline_pages(unsigned long start_pfn,
 	ret = 0;
 	if (drain) {
 		lru_add_drain_all();
-		flush_scheduled_work();
 		cond_resched();
 		drain_all_pages();
 	}
@@ -862,7 +861,6 @@ static int offline_pages(unsigned long start_pfn,
 	}
 	/* drain all zone's lru pagevec, this is asyncronous... */
 	lru_add_drain_all();
-	flush_scheduled_work();
 	yield();
 	/* drain pcp pages , this is synchrouns. */
 	drain_all_pages();