wpa_supplicant: remove gnutls support

wpa_supplicant isn't API compatible with gnutls3 so remove support.
It's probably hardly used since openssl is far more common, and
wpa_supplicant can use its internal routines if it's not available.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>