Remove widgets CSS as hybrid-core-widgets has been removed

Since Hybrid Core no longer uses `hybrid-core-widgets`, the CSS added for it should be safe to remove (?).

The only thing I can think of is if a theme is using the classes in a custom widget. But if the theme is using 2.0, there should be no reason to still have theme support added for `hybrid-core-widgets` and the styling is only enqueued when `current_theme_supports( 'hybrid-core-widgets' )` is true.