btrfs: dev stats item key conversion per cpu type is not needed

%found_key is not used, drop it since it hasn't been used since the
beginning in 733f4fbb ("Btrfs: read device stats on mount, write
modified ones during commit").

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