Delete unnecessary fields in the ButtonStyle class.
Delete unnecessary fields in the ButtonStyle class.

These fields are inherited properly, just like in other Style classes.
ipython/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,jupyter-widgets/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets