message: API: Send empty response on notification deletion
  Previously the deleted notification was sent as response, without proper
  data sanitation. Since the response isn't actually used by the front-end
  anyway, this vulnerability is solved here by simply switching to an empty
  success response object.
diff: @@ -249,28 +249,11 @@ export class NotificationController {
  * @apiPermission admin
  *
  * @apiParam {String} id Notification ID.
  - * @apiParam {IUser} currentUser Currently logged in user.
  *
  - * @apiSuccess {Object} deletion Object with deleted notification.
  + * @apiSuccess {Object} result Empty object.
  *
  * @apiSuccessExample {json} Success-Response:
  - * {
  - * "$__": {...},
  - * "isNew": false,
  - * "_doc": {
  - * "__v": 0,
  - * "user": {...},
  - * "text": "Course ProblemSolver has an updated text unit.",
  - * "isOld": false,
  - * "changedCourse": {...},
  - * "changedLecture": {...},
  - * "changedUnit": {...},
  - * "createdAt": "2018-03-22T00:42:12.577Z",
  - * "updatedAt": "2018-03-22T00:42:12.577Z",
  - * "_id": {...}
  - * },
  - * "$init": true
  - * }
  + * {}
  *
  * @apiError NotFoundError Notification could not be found.
  */
  @@ -281,6 +264,7 @@ export class NotificationController {
  if (!notification) {
  throw new NotFoundError('Notification could not be found.');
  }
  - return notification.remove();
  + await notification.remove();
  + return {};
  }
  }
  