Always respect the user's bar_width setting

Resolves https://github.com/kivy/kivy/issues/1669

I understand why the code was originally there; since the default bar_width is 2, if the user selects to scroll only on bars we would want the bars to be wider. But just overwriting bar_width to 9 is not the solution. Instead the user should have set the default bar_width to something else.