cfg80211: remove cleanup_work kernel-doc

I evidently forgot this when removing the work itself.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/cfg80211.h

@@ -2869,7 +2869,6 @@ struct cfg80211_cached_keys;
  * @mgmt_registrations_lock: lock for the list
  * @mtx: mutex used to lock data in this struct, may be used by drivers
  *	and some API functions require it held
- * @cleanup_work: work struct used for cleanup that can't be done directly
  * @beacon_interval: beacon interval used on this device for transmitting
  *	beacons, 0 when not valid
  * @address: The address for this device, valid only if @netdev is %NULL