iwlwifi: remove notif_missed_beacons variable

This patch removes notif_missed_beacons from priv since it was never
used. Missed beacons notification will have only meaning if roaming
will be implemented in mac80211.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>