dm vdo: remove bad check of bi_next field

Remove this check to prevent spurious warning messages due to
the behavior of other storage layers. This check was intended
to make sure dm-vdo does not chain metadata bios together.
However, vdo has no control over underlying storage layers,
so the assertion is not always true.

Signed-off-by: Ken Raeburn <raeburn@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>