wifi: cfg80211/mac80211: remove more 5/10 MHz code

We still have ieee80211_chandef_rate_flags() and all that,
but all the users seem pretty much broken (deflink, etc.)
Remove all the code. It's been two years since last anyone
even vaguely entertained the notion of looking at this and
fixing it.

Link: https://patch.msgid.link/20250329221419.c31da7ae8c84.I1a3a4b6008134d66ca75a5bdfc004f4594da8145@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>