Remove extra class from CSS

Remove `report-block-list-issue-parent` class
from CSS now that it is no longer used.

Add a vaildator for the nested level in the issues
list component to make sure it is 0, 1, or 2.

app/assets/javascripts/reports/components/issues_list.vue

@@ -71,6 +71,7 @@ export default {
       type: Number,
       required: false,
       default: 0,
+      validator: (value) => [0, 1, 2].includes(value),
     },
   },
   computed: {

app/assets/stylesheets/page_bundles/reports.scss

@@ -5,11 +5,6 @@
     max-height: 170px;
     overflow: auto;
   }
-
-  .report-block-list-issue-parent {
-    padding: $gl-padding-top $gl-padding;
-    border-top: 1px solid var(--border-color, $border-color);
-  }
 }
 
 .report-block-container {