net80211: remove (optional) RSN from preobereq

802.11i-2004 did not specify that RSN can be added to the probereq,
only to the proberesp (and others) yet it was added in what seems was
b032f27c365b992e9d8e42214183b39acfb8c6ac (multi-BSS support).
802.11-2020 does not list the IE either for probereq so remove it.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Okayed by: adrian
Differential Revision: https://reviews.freebsd.org/D34893