Remove bottom border from Issuable titles

Prevent duplicate lines when issue/merge request is empty.

Empty MR

Empty Issue

app/assets/stylesheets/pages/detail_page.scss

@@ -20,11 +20,10 @@
 
 .detail-page-description {
   .title {
-    margin: 0 0 16px;
+    margin: 0;
     font-size: 2em;
     color: $gl-gray-dark;
-    padding: 0 0 0.3em;
-    border-bottom: 1px solid $white-dark;
+    padding: 0;
   }
 
   .description {