remove the label_attr option which is not available in the button type
minor #3651 [Reference][Forms] remove the label_attr option which is not available in the button type (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

[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.

Commits
-------

5bafc3c remove the label_attr option which is not available in the button type