[Reference][Forms] remove the label_attr option which is not available in the button type

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | 

The ``label_attr`` is neither defined in the ``ButtonType`` class nor in the ``BaseType`` class, but in the ``FormType`` class instead. Thus, it can't be used for the ``button`` type.