I want to delete only note "leave reply" in single post.
yes, you can do this with the dashboard options for selected single posts.
I checked this with Twenty Twelve, and there is no trace of 'Leave a Reply' left in a single post if the comments are 'un-allowed'.
particular deleting your mentioned section from single.php of Twenty Twelve will not leave any output.
I do need comments on my blog
this is a direct contradition of your attempt to remove the comments code from single.php;
'blog' equals 'posts' and the commenting is usually done via the single post.
if your sitution is different, please post a link to your site to illustrate your problem, and to allow somebody to start investigating.