wpa_supplicant.conf.5: Remove removed variables

2005-09-25 - wpa_supplicant v0.4.5 removed "server_nai"
2008-02-22 - wpa_supplicant v0.6.3 removed "eappsk" and "nai"

PR: 284126
MFC after: 3 days
Reviewed by: carlavilla, ziaee
Reported by: J.R. Oldroyd <fbsd@opal.com>
Differential Revision: https://reviews.freebsd.org/D49010