iwlwifi: mvm: remove stray nd_config element

When the netdetect debugfs entry was removed, the nd_config element
was accidentally left in the iwl_mvm structure.  Remove it.

Fixes: dbb04b0d ("iwlwifi: mvm: remove netdetect debugfs entry")
Reported-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>