Remove broken marker centreing code

Apparently, having an image's origin be in the center is too
advanced of a concept for The Web™. I guess we can't do this then.

overviewer_core/data/web_assets/overviewer.css

@@ -173,9 +173,3 @@ div.worldcontrol select {
     padding: 0 5px;
     color: #333;
 }
-
-.custom-icon {
-    position:relative;
-    left:-50%;
-    top:-50%;
-}