mac80211: remove description of dropped member

Commit 976bd9ef

 ("mac80211: move beacon_loss_count into ifmgd")
removed the member from the sta_info struct but the description stayed
lingering. Remove it.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>