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.com


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