hostapd: remove gnutls support

hostapd isn't API compatible with gnutls3 so remove support.
It's probably hardly used since openssl is far more common, and hostapd
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>