FIX: `z` helper doesn't resolve inside `calc`

It resulted in invalid `z-index: calc(z("tooltip") - 100)` in the output CSS. `calc` wasn't necessary here anyway since both operands are constant.

app/assets/stylesheets/common/base/d-popover.scss

@@ -75,7 +75,7 @@ $d-popover-border: $primary-medium;
     color: transparent;
     content: "";
     position: absolute;
-    z-index: calc(z("tooltip") - 100);
+    z-index: z("tooltip") - 100;
   }
 
   .d-popover-top-arrow {