Strip HTML Tags From Search Results

I don’t know about the code you are trying to use, personally I would never use a solution that excludes results, it’s very possible the item being searched for is in those results and excluding them is only a degradation of service.

Instead you can use the search.php template file to adjust how your search results are displayed, there is a very handy function available in php called strip_tags() which will remove all HTML tags from a string.

UPDATE:

Specifically:

search.php:

<?php
/**
 * Template Name: Search Results Page
 */
get_header();
?>
<section id="primary">
    <div id="content" role="main">
        <?php if (have_posts()): ?>
            <header class="page-header"><?php _e("Search Results"); ?></header>
            <div class="entry-content">
                <?php
                while (have_posts()): the_post();
                    echo strip_tags(get_the_content());
                endwhile;
                ?>
            </div>
        <?php endif; ?>
    </div>
</section>
<?php
get_footer();