f2fs: remove writeback_inodes_sb in f2fs_remount

Since sync_inodes_sb has been used, there is no need to
use writeback_inodes_sb, so remove it.

Signed-off-by: Liu Song <liu.song11@zte.com.cn>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>