How to remove “Read on” content in the_excerpt?

If you’re using the Twenty Eleven theme I think you need to remove that theme’s filter before you can define your own:

remove_filter( ‘excerpt_more’, ‘twentyeleven_auto_excerpt_more’ );

edit building from t-p

try this:

    add_action( 'after_setup_theme', 'my_child_theme_setup' );
    function my_child_theme_setup() {
       remove_filter( 'excerpt_more', 'twentyten_auto_excerpt_more' );
    }

In case you are using twentyeleven, use “twentyeleven_auto_excerpt_more” instead of ‘twentyten_auto_excerpt_more’