md/raid5: remove an incorrect assert in in_chunk_boundary

Now that the original bdev is stored in the bio this assert is incorrect
and will trigger for any partitioned raid5 device.

Reported-by: Florian Dazinger <spam02@dazinger.net>
Tested-by: Florian Dazinger <spam02@dazinger.net>
Cc: stable@vger.kernel.org # 5.12
Fixes: 309dca30

 ("block: store a block_device pointer in struct bio"),
Reviewed-by: Guoqing Jiang <jiangguoqing@kylinos.cn>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Song Liu <song@kernel.org>