wan: cosa: remove dead cosa_net_ioctl() function

The ndo_do_ioctl callback is never called with the COSAIO* commands,
so this is never used. Call the hdlc_ioctl function directly instead.

Any user space code that relied on this function working as intended
has never worked in a mainline kernel since before linux-1.0.

Cc: "Jan \"Yenya\" Kasprzak" <kas@fi.muni.cz>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>