slcan: remove unused 'leased', 'line' and 'pid' fields from the 'slcan' structure

Signed-off-by: Matvejchikov Ilya <matvejchikov@gmail.com>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>