Removing from Comment

wp_list_comments() accepts a walker in the array of its first parameter. This is a class that renders the output. If you don’t provide one, the default class will be used, Walker_Comment. You can find it in wp-includes/comment-template.php.

To change the complete comment list out, create a custom walker in your functions.php which extends the default class:

class WPSE_127257_Walker_Comment extends Walker_Comment
{
    function start_lvl( &$output, $depth = 0, $args = array() ) {
        // do nothing.
    }
    function end_lvl( &$output, $depth = 0, $args = array() ) {
        // do nothing.
    }
    function end_el( &$output, $comment, $depth = 0, $args = array() ) {
        // do nothing, and no </li> will be created
    }
    protected function comment( $comment, $depth, $args ) {
        // create the comment output
        // use the code from your old callback here
    }
}

And then you use that class when you call wp_list_comments():

wp_list_comments(
    array (
        'walker' => new WPSE_127257_Walker_Comment
    )
);