mac80211: clean up station debugfs

Remove items that can be retrieved through nl80211. This also
removes two items (tx_packets and tx_bytes) where only the VO
counter was exposed since they are split up per AC but in the
debugfs file only the first AC was shown.

Also remove the useless "dev" file - the stations have long
been in a sub-directory of the netdev so there's no need for
that any more.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>