Revert #23108 to restore list group borders (#24411)

* revert #23108, which had no bug but was a problem with the example code

* add example to the docs for future proofing

* move it up

docs/4.0/components/card.md

@@ -90,6 +90,19 @@ Create lists of content in a card with a flush list group.
 </div>
 {% endexample %}
 
+{% example html %}
+<div class="card" style="width: 20rem;">
+  <div class="card-header">
+    Featured
+  </div>
+  <ul class="list-group list-group-flush">
+    <li class="list-group-item">Cras justo odio</li>
+    <li class="list-group-item">Dapibus ac facilisis in</li>
+    <li class="list-group-item">Vestibulum at eros</li>
+  </ul>
+</div>
+{% endexample %}
+
 ### Kitchen sink
 
 Mix and match multiple content types to create the card you need, or throw everything in there. Shown below are image styles, blocks, text styles, and a list group—all wrapped in a fixed-width card.

scss/_list-group.scss

@@ -86,13 +86,8 @@
 .list-group-flush {
   .list-group-item {
     border-right: 0;
-    border-bottom: 0;
     border-left: 0;
     border-radius: 0;
-
-    &:first-child {
-      border-top: 0;
-    }
   }
 
   &:first-child {