iwlwifi: remove wrong key use check

Default WEP keys and regular keys are not allocated
from the same space in the firmware, so we shouldn't
use the same bits to indicate in use.

For default WEP keys, however, mac80211 won't allow
using the same key index twice, so the check is not
necessary at all -- add/remove are perfectly nested
due to those checks.

Other keys are allocated in the device in a global
array that only has a certain number of slots, so
for that we need to keep the allocation bitmap; but
the 802.11 key index isn't relevant there.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>