Only Show Primary Category in Single Post

/* Only Show Primary Category in Single Post */
function only_show_primary_category( $thelist ) {
    if ( is_single() && is_singular() ) {
        $cat_id = jnews_get_primary_category( get_the_ID() );
        $cat = get_term( $cat_id, 'category' );
        $primary_cat = '<a href="' . esc_url( get_category_link( $cat->term_id ) ) . '" rel="category tag">' . $cat->name . '</a></li>';
        return $primary_cat;
    }

    return $thelist;
}
add_filter( 'the_category', 'only_show_primary_category', 999 );