Delete or edit postmeta with specific meta_key?

Untested, but this might be what you’re looking for:

$args = array( 'fields'          => 'ids',
               'posts_per_page'  => -1,
               'post_type'       => 'attachment',
               'meta_key'        => 'background-video'
               );
$all_ids = new WP_Query( $args );
if ( $all_ids->have_posts() ) {
    while ( $all_ids->have_posts() ) {
        $all_ids->the_post();
        update_post_meta( get_the_ID(), 'background-video', '' );
    }
}
wp_reset_postdata();

References

• WP_Query » Usage » Standard Loop