iwlwifi: remove set_ct_kill operation

This operation is only ever called from set_hw_params,
which is also already based on the config/ops, so that
there's no need to have a separate set_ct_kill op and
we can just call the right ct_threshold function.

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