In `wp-includes/comment-template.php`, remove dead code:

* In `get_comment_reply_link()`, `$link` is set twice before it is used.
* In `Walker_Comment::start_lvl()`, `case 'ul':` is unreachable unless placed before `default:`. 
* In `Walker_Comment::end_lvl()`, `case 'ul':` is unreachable unless placed before `default:`.
* In `comment_form()`, `$id` is conditionally set and never used. 

See #27882.


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