Remove user rights (task CE-729)

Removing global user rights + asynchronous task, which removes all user's rights from all Wikias.
You can bypass using AsyncTaskList directly in a lot of cases, including here. We can just do:
```php
$task = new RemoveUserRightsTask();
$task->call(...);
$task->queue();
```
This is a bit easier to read and has the same effect. The only case in which we should really be using AsyncTaskList directly is when we need a task that encompasses calls from multiple BaseTask objects.