Remove celery code

[michaelbukachi]

When going through the code I noticed that it requires celery:

   if self.screen_content.get('synchronous', False):
            http_task.delay(request_conf=http_request_conf) # Here
        else:
            self.make_request(
                http_request_conf=http_request_conf,
                response_session_key_save=self.screen_content['session_key'],
                session=self.ussd_request.session,
                logger=self.logger
            )

Is it possible to configure a callback so that a user can configure there own async implementation?
I'm willing to do a PR for this.