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: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>