Programmatically remove RSS footer from another plugin

I’m writing a plugin that modifies the content of the RSS feed. I’d like to remove the RSS footer content from the WordPress SEO plugin, from my plugin. This code doesn’t work:

// Prevent WordPress SEO RSS footer
add_action( 'init', 'mediarss_seotweak', 11 );

function mediarss_seotweak() {
	remove_filter( 'the_content_feed', array( WPSEO_Frontend, 'embed_rssfooter' ) );
	remove_filter( 'the_excerpt_rss', array( WPSEO_Frontend, 'embed_rssfooter_excerpt' ) );
}

Appreciate any help to point me in the right direction. Thanks

http://wordpress.org/plugins/wordpress-seo/