We haven't used this function for years, since commit c781944b
("cfg80211: Remove unused cfg80211_can_use_iftype_chan()") which
itself removed a function unused since commit 97dc94f1
("cfg80211: remove channel_switch combination check"), almost eight
years ago.

Also remove the now unused enum cfg80211_chan_mode and some struct
members that were only used for this function.

Link: https://lore.kernel.org/r/20220412220958.1a191dca19d7.Ide4448f02d0e2f1ca2992971421ffc1933a5370a@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>