Do not overwrite the updateModel method of a FormComponent

- if you use wicket the right way there is no need the overwrite the updateModel method for a custom form component
- this strange code inside the updateModel method caused an error in combination with the wicket-bootstrap library
  -> the FormGroup component implements the interface IFormModelUpdateListener (which is also wrong) and cause that the updateModel method of each FormComponent gets called twice
  -> calling this updateModel method twice within one request cycle results in an empty collection for the model object