cfg80211: drop duplicated documentation of field "_net"

The field "_net" was already documented above the definition of struct
wiphy. Both comments were identical.
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200221115604.594035-4-Jerome.Pouiller@silabs.comSigned-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/cfg80211.h

@@ -4699,7 +4699,6 @@ struct wiphy {
 
 	struct list_head wdev_list;
 
-	/* the network namespace this phy lives in currently */
 	possible_net_t _net;
 
 #ifdef CONFIG_CFG80211_WEXT