Change markdown header tab anchor links to buttons

What does this MR do?

Changes markdown header tab anchor links to buttons. (see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21988#note_105636972)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

app/assets/stylesheets/framework/markdown_area.scss

@@ -66,32 +66,6 @@
         padding-top: 10px;
       }
     }
-
-    //li.md-header-tab {
-    //  button {
-    //    width: 100%;
-    //    font-size: 14px;
-    //    line-height: 19px;
-    //    color: $gl-text-color-secondary;
-    //    padding: $gl-btn-padding;
-    //    padding-top: 0;
-    //    border: 0;
-    //    border-bottom: 2px solid transparent;
-    //
-    //    &:hover {
-    //      color: $black;
-    //      border-color: $gray-darkest;
-    //    }
-    //  }
-    //
-    //  &.active {
-    //    button {
-    //      font-weight: 600;
-    //      color: $black;
-    //      border-bottom: 2px solid $theme-gray-800;
-    //    }
-    //  }
-    //}
   }
 }