remove attr_accessor,  not compatible with OpenStruct
remove attr_accessor,  not compatible with OpenStruct

Need to remove `attr_accessor` on `user_id` and `room_id`.

Due to these `attr_accessor`, `user_id` and `room_id` were not accessible:

```ruby
b = Babili::Platform::Membership.new(room_id: room_id, user_id: user_id)
b.user_id # return nil
```
Spin42/babili-ruby