Clean up CSS for buttons in Bartik

[Note: I'm not sure if there's a task issue that lists these kind of changes. I did find some issues about cleaning up CSS but not specifically about removing classes that can simply be replaced by CSS2 selectors (that did not work in IE6),

With the drop of IE6 support (#308865: Drop IE6 support in Drupal core), there is no need anymore to use a class to indicate that a button is disabled, as both CSS and JS can easily be replaced:

CSS:

/* old */
input.form-button-disabled {... }
/* new */
input[type=submit][disabled] { ... }

JS with jQuery:

// old
$('.form-button-disabled').click(...);
// new
$('input[type=submit][disabled]').click(...);

You could argue that this holds for many (many) classes that are added by Drupal, (e.g. the class "form-button" used in the example above), but the class "form-button-disabled" is added based on an attribute that is likely to be dynamically changed using javascript (after some conditions have been met) that easily can leave the class on the element.

If the class is needed for some other reason, then at least the CSS should be changed in the Seven and Bartik themes.