If you want to learn CSS for this site, I'd suggest first getting a Neocities or a local web server so you can learn how HTML and CSS work when you aren't working around someone else's code (W3Schools has a course on this), and also learn how to find elements on a page and their CSS with your web browser's Inspect tool.

Using Inspect, I was able to figure out this code:

img.banner.shadow {
  box-shadow: none;
}

You could also remove all shadows on your profile by setting your shadow color in profile settings to #0000.