md: clean up accounting for issued sync IO

It's no longer used and can be removed, also remove the field
'gendisk->sync_io'.

Link: https://lore.kernel.org/linux-raid/20250506124903.2540268-10-yukuai1@huaweicloud.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Xiao Ni <xni@redhat.com>