Btrfs: remove misleading handling of missing device scrub

scrub_submit() claims that it can handle a bio with a NULL block device,
but this is misleading, as calling bio_add_page() on a bio with a NULL
->bi_bdev would've already crashed. Delete this, as we're about to
properly handle a missing block device.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>