WP Core actually uses Open Sans font; it’s not a plugin. there is a plugin that removes it, but you can probably simply dequeue or deregister it.
adding this to functions.php should work; if you want it removed from the backend as well, hook into the admin_print_styles
action.
function dequeue_opensans_css() {
wp_dequeue_style( 'open-sans' );
}
add_action( 'wp_enqueue_scripts', 'dequeue_opensans_css', 999 );
if it’s in the footer, change the add_action()
line:
add_action('wp_footer','dequeue _opensans_css',1);
untested, but that should work.