btrfs: remove unused member btrfs_device::work

This is a leftover from recently removed bio scheduling framework.

Fixes: ba8a9d07 ("Btrfs: delete the entire async bio submission framework")
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.h

@@ -120,8 +120,6 @@ struct btrfs_device {
 	/* per-device scrub information */
 	struct scrub_ctx *scrub_ctx;
 
-	struct btrfs_work work;
-
 	/* readahead state */
 	atomic_t reada_in_flight;
 	u64 reada_next;