Put revoke for canceling upload task behind ``enable_celery_tasks`` flag

class DefaultJobDispatcher:

    def __init__(self, app):

    def __init__(self, app: MinimalManagerApp):

        self.app = app

        self.job_runners = self.app.job_config.get_job_runner_plugins(self.app.config.server_name)

        # Once plugins are loaded, all job destinations that were created from

        # The runner name is not set until the job has started.

        # If we're stopping a task, then the runner_name may be

        # None, in which case it hasn't been scheduled.

        if job.tool_id == "__DATA_FETCH__":

        if self.app.config.enable_celery_tasks and job.tool_id == "__DATA_FETCH__":

            from galaxy.celery import celery_app

            celery_app.control.revoke(job.job_runner_external_id)