wan: remove stale Kconfig entries

The dscc4 driver was removed in 2019 but these Kconfig entries remain,
so remove them as well.

Fixes: 28c9eb90

 ("net/wan: dscc4: remove broken dscc4 driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>