mac80211: remove final sta_info_flush()

When all interfaces have been removed, there can't
be any stations left over, so there's no need to
flush again. Remove this, and all code associated
with it, which also simplifies the function.

Tested-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>