drivers: wifi: winc1500: remove unused send_cb and send_user_data

Both 'cb' and 'user_data' parameters for send/sendto were saved as
'send_cb' and 'send_user_data' members in socket information. None of
them are actually used, so drop them.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>