Fix label list divider (#25312)

We only needs 2 lines to hide the dividers.

```
  $dropdownLabelFilter.dropdown('setting', {'hideDividers': 'empty'});
  $dropdownLabelFilter.dropdown('refreshItems');
```

Other code blocks are refactored by the way.


![image](74989996-fcea-4df4-b534-b06f7957939a)


![image](ee3b3761-b96e-4fb5-b646-e9d3117e5f40)

web_src/js/features/repo-issue.js

@@ -149,6 +149,7 @@ export function initRepoIssueSidebarList() {
       }
     }
   });
+  $('.ui.dropdown.label-filter').dropdown('setting', {'hideDividers': 'empty'}).dropdown('refreshItems');
 }
 
 export function initRepoIssueCommentDelete() {