btrfs: remove unused btrfs_device::flush_bio_sent

The status of flush bio is tracked as a status bit, changed in commit
1c3063b6 ("btrfs: cleanup device states define
BTRFS_DEV_STATE_FLUSH_SENT"), the flush_bio_sent was forgotten.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>