Switch to default core markup for search form, comment form, and comments

I don't know how many WordPress versions `_s` supports but it would be nice to add support for the new core HTML5 markup. 

`add_theme_support( 'html5', array( 'search-form', 'comment-form', 'comment-list' ) );`

This pull request adds the above code, and:
* Removes the `_s_comment` callback and function
* Removes `searchform.php`