You can find settings like this using the browser Developer Tools. (F12 on most browsers.)
I was able to turn it off using Styles | Advanced Stylesheet (available at least on 8.1.25) and adding the following CSS.
.ia_table__headContainer, .ia_table__footContainer {
box-shadow: none;
}
This will affect all tables in the project.
There's probably a way of injecting this into one table but I haven't time to figure it out.
