iwlagn: remove a bogus AGG_OFF check

Even if this check were to happen, using the
txq_id here (which is a HW queue) would lead
to confusion in mac80211. Luckily, it doesn't
seem like this can ever happen.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>