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@changeid


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