Removing non native customizer settings from a child theme

I am guessing a lot about how your themes work but, the child theme’s functions.php runs before the parent theme’s functions.php, so anything loaded by the parent’s functions isn’t going to be present for you to remove. You need to hook your function so that it runs later.

add_action('after_setup_theme', 'remove_custom');

Though you would need to mess about with the $wp_customize variable. It would be easier to hook into the customize_register hook with a high priority number so that your code runs after the parent registration code.

add_action( 'customize_register', 'remove_custom', 1000 );