Remove waiting users memberships

We'll use the `:waiting_user` role instead.

![](http://www.lovethispic.com/uploaded_images/9765-Lighter-Flame.gif)

ToDo:
 - [ ] Check if a user is already a group member before adding him to a waiting list
 - [ ] Get an ordered waiting list for a Group (`asc` by `updated_at`)
 - [ ] Add specs for `WaitingUsersController`
This is not valid anymore because `waiting_user`is a role stored in `Membership` model just like `admin` and `member` roles.