fix(list): remove unnecessary z-index (#7678)

**Related Issue:** #7599

## Summary

- removes z-index on a container within the list component

Author: driskull

packages/calcite-components/src/components/list-item/list-item.scss

@@ -55,7 +55,7 @@ td {
 
 tr:focus,
 td:focus {
-  @apply focus-inset z-sticky;
+  @apply focus-inset;
 }
 
 .content,

packages/calcite-components/src/components/list/list.scss

@@ -13,9 +13,7 @@
   flex
   w-full
   flex-col
-  bg-transparent
-  relative
-  z-default;
+  bg-transparent;
   * {
     @apply box-border;
   }