Whenever you change the slug of a permalink on posts ( and custom post types with post capabilities ), WordPress will save the old slug in the post_meta table with the key _wp_old_slug. On a normal installation the old slug should redirect to the new slug using wp_old_slug_redirect(). There are a couple ways to remove them, the easiest is probably SQL directly:
/**
* Delete posts old slugs from database in WordPress
*/
function delete_old_slugs() {
global $wpdb;
$wpdb->get_results( "DELETE FROM wp_postmeta WHERE meta_key = '_wp_old_slug'" );
}
delete_old_slugs();
You may also be able to find a plugin to do the work for you, the first one that I could find is Remove Old Slugs plugin which will also supply you a list of the old slugs which seems neat.