net80211: cleanup ic_flags / iv_flags

Flags

• IEEE80211_F_ASCAN
• IEEE80211_F_SIBSS
• IEEE80211_F_IBSSON

are not used since r170530 (old WI compatibility ioctls removal)
and r178354 (removed from other places).

IEEE80211_F_TXPOW_FIXED was never utilized; initially added with
IEEE80211_F_TXPOW_AUTO / IEEE80211_F_TXPOW_OFF flags,
which were replaced with IEEE80211_C_TXPMGT capability check in r138568.

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D9369