code: def _del_sub_prop(self, key):
          
          _helpers._del_sub_prop(self._properties, [self._job_type, key])
docs: Remove ``key`` from the ``self._properties[self._job_type]`` dict.
  
          Most job properties are inside the dictionary related to the job type
          (e.g. 'copy', 'extract', 'load', 'query'). Use this method to clear
          those properties::
  
              self._del_sub_prop('useLegacySql')
  
          This is equivalent to using the ``_helper._del_sub_prop`` function::
  
              _helper._del_sub_prop(
                  self._properties, ['query', 'useLegacySql'])
  
          Arguments:
              key (str):
                   Key to remove in the ``self._properties[self._job_type]``
                   dictionary.