cfg80211: remove CFG80211_MAX_NUM_DIFFERENT_CHANNELS

We no longer need to put any limits here, hardware will and
mac80211-hwsim can do whatever it likes. The reason we had
this was some accounting code (still mentioned in the comment)
but that code was deleted in commit c781944b ("cfg80211:
Remove unused cfg80211_can_use_iftype_chan()").

Link: https://lore.kernel.org/r/20210506221159.d1d61db1d31c.Iac4da68d54b9f1fdc18a03586bbe06aeb9515425@changeid


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