Remove $zindex-dropdown-backdrop from docs (#23461)

It was removed in 6d64afe5.

docs/4.0/layout/overview.md

@@ -162,7 +162,6 @@ Several Bootstrap components utilize `z-index`, the CSS property that helps cont
 We don't encourage customization of these values; should you change one, you likely need to change them all.
 
 ```scss
-$zindex-dropdown-backdrop:  990 !default;
 $zindex-dropdown:          1000 !default;
 $zindex-fixed:             1030 !default;
 $zindex-sticky:            1030 !default;