You can get rid of this code
<?php if(get_the_post_thumbnail()): ?>
<media:content url="<?php $image = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'medium'); echo $image[0]; ?>" medium="image" />
<?php endif; ?>
That should remove the image associated with the post. It is checking to see if the featured image is set, and then if it is, it displays it. Removing the code, removes the image from display.