How To Delete It?

It might be easier if you stick to a theme from the WordPress theme repository for this conversation.

http://ww.wp.xz.cn/extend/themes/

Especially as Esmi pointed out already that commercial products aren’t supported here.

Generically speaking it’s in your theme’s comments.php file and is part of the comment_form() arguments.

http://codex.ww.wp.xz.cn/Function_Reference/comment_form

Make a child theme and work with a copy of the comments.php file if it exists and work with that function.