Btrfs: remove batch plug in run_scheduled_IO

Block layer has a limit on plug, ie. BLK_MAX_REQUEST_COUNT == 16, so
we don't gain benefits by batching 64 bios here.
Signed-off-by: NLiu Bo <bo.li.liu@oracle.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -517,12 +517,6 @@ static noinline void run_scheduled_bios(struct btrfs_device *device)
 					 &device->work);
 			goto done;
 		}
-		/* unplug every 64 requests just for good measure */
-		if (batch_run % 64 == 0) {
-			blk_finish_plug(&plug);
-			blk_start_plug(&plug);
-			sync_pending = 0;
-		}
 	}
 
 	cond_resched();