This element (class="skeleton-wrapper") is responsible for the display of this gray container and as you can see, it’s not a part of the Slider widget:
If you remove this element form your website, the grey container above the Slide will disappear.
As for the issue caused because of the updated classed, I understand your frustration and I am really sorry for this inconvenience!
However, you see, temporary classes are auto-generated, can change at any time, and may break your customization. Thus, we recommend you using the Stable classes. They are predictable and won’t change over time, which makes them perfect for styling with CSS.
Feel free to check our CSS guide where you can find more info about stable and temporary classes 
- Guide to CSS Codes