The ppp_set_auth() is guard by #if PPP_AUTH_SUPPORT in lwIP, so
make it consistent. This also simplify the code a bit because the code
in #if PAP_SUPPORT guard and #if CHAP_SUPPORT guard are exactly the same.
Once NETIF_PPP_AUTHTYPE_NONE added to esp_netif_auth_type_t, it also allows
setting NETIF_PPP_AUTHTYPE_NONE with this change.
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Merges https://github.com/espressif/esp-idf/pull/4639
