wifi: mac80211: remove unnecessary struct forward declaration

This just causes kernel-doc to complain at this spot, but
isn't actually needed anyway, so remove it.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230827135854.33a5591dfdeb.If4e7e1a1cb4c04f0afd83db7401c780404dca699@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/ieee80211_i.h

@@ -625,7 +625,6 @@ struct ieee80211_if_ocb {
  * vendor-specific mesh synchronization
  *
  */
-struct ieee802_11_elems;
 struct ieee80211_mesh_sync_ops {
 	void (*rx_bcn_presp)(struct ieee80211_sub_if_data *sdata, u16 stype,
 			     struct ieee80211_mgmt *mgmt, unsigned int len,