iwlwifi: remove monitor check

Off-channel reception is acceptable in monitor
mode, and checking for monitor mode this way is
not really correct anyway since it could be the
case while operating.

Now iwl_is_monitor_mode() is no longer used so
remove it completely.

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