mac80211: remove unneeded ieee80211_run_deferred_scan()

Ilan pointed out to me that ieee80211_start_next_roc()
already calls ieee80211_run_deferred_scan() if the list
of ROC items is empty, so there's no need to call it
again after calling ieee80211_start_next_roc().

Reported-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>