Remove struct ieee80211_mgmt::u.probe_req

Remove struct ieee80211_mgmt::u.probe_req

This struct in the union is empty, but the design of using a zero-length
u8 array here is not fully compatible with C++ and can result in
undesired compiler warnings. This struct is not used anymore, so it can
be removed from the struct ieee80211_mgmt definition to complete the
changes started in commit d447cd596f0a9f73850229e7fa2bdd35755dc750
('Updates for stricter automatic memcpy bounds checking').

Signed-off-by: Jouni Malinen <j@w1.fi>