remove alias from apidoc Body Parameters for creating a challenge task

https://habitica.com/apidoc/#api-Task-CreateChallengeTasks

Aliases can't be added by the challenge creator as shown by this test:
2dad540d43/test/api/v3/integration/tasks/challenges/POST-tasks_challenge_id.test.js (L77-L87)

That's a requirement because in each user's account, each alias must
be unique, and you can't enforce that if the challenge creator can
assign an alias.

website/server/controllers/api-v3/tasks.js

@@ -247,7 +247,6 @@ api.createUserTasks = {
  * @apiParam (Body) {String} text The text to be displayed for the task
  * @apiParam (Body) {String="habit","daily","todo","reward"} type Task type, options are: "habit",
  *                                                                "daily", "todo", "reward".
- * @apiParam (Body) {String} [alias] Alias to assign to task
  * @apiParam (Body) {String="str","int","per","con"} [attribute] User's attribute to use,
  *                                                               options are: "str",
  *                                                               "int", "per", "con".