mac80211: Remove unused variable in per STA debugfs struct

Remove unused variable in per STA debugfs structure, 'commit 34e89507
("mac80211: allow station add/remove to sleep")' removed the only user of
'add_has_run'.

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qti.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>