Just a bit of info for others who wonder what the default styles is. There is no reset option to remove it.
I am working on a clients site improving performance scores and come across several grids with this style. As you can see there is no CSS class name and I thought it might be some legacy system.
In short, it comes from using the pre-defined Webflow Grid.
The other predefines have an easy to identify icon to distinguish where as the grids are the same.
Hope this helps others.