code: def delete_session(self, client_id):
          
          try:
              session = self._sessions[client_id][0]
          except KeyError:
              session = None
          if session is None:
              self.logger.debug("Delete session : session %s doesn't exist" % client_id)
              return
  
          # Delete subscriptions
          self.logger.debug("deleting session %s subscriptions" % repr(session))
          self._del_all_subscriptions(session)
  
          self.logger.debug("deleting existing session %s" % repr(self._sessions[client_id]))
          del self._sessions[client_id]
docs: Delete an existing session data, for example due to clean session set in CONNECT
          :param client_id:
          :return: