btrfs: remove unused btrfs_device::flush_bio_sent

The status of flush bio is tracked as a status bit, changed in commit
1c3063b6db ("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>

fs/btrfs/volumes.h

@@ -82,7 +82,6 @@ struct btrfs_device {
 
 	unsigned long dev_state;
 	blk_status_t last_flush_error;
-	int flush_bio_sent;
 
 #ifdef __BTRFS_NEED_DEVICE_DATA_ORDERED
 	seqcount_t data_seqcount;