Snippet to remove the Additional CSS section from the Customizer. This section is introduced in WordPress 4.7.

functions.php

<?php

add_action( 'customize_register', 'prefix_remove_css_section', 15 );
/**
 * Remove the additional CSS section, introduced in 4.7, from the Customizer.
 * @param $wp_customize WP_Customize_Manager
 */
function prefix_remove_css_section( $wp_customize ) {
	$wp_customize->remove_section( 'custom_css' );
}