mac80211: remove sta_info_flush() from interface teardown

All interface types now properly clean up their stations
using some form of sta_info_flush() themselves, so there's
no need to try it again at teardown. Remove the call to
get rid of the extra delay from the synchronize_net() and
rcu_barrier() calls.

Reported-by: Moshe Benji <moshe.benji@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>