btrfs: remove stale comment for argument seed of btrfs_find_device

Commit b2598edf ("btrfs: remove unused argument seed from
btrfs_find_device") removed the argument seed from btrfs_find_device
but forgot the comment, so remove it.

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