How to remove the customizer CSS option

Now that WordPress 4.7 has been released, most people will notice an “Additional CSS” section in the customizer when they update. This is an awesome feature and a perfect use of the customizer. It’ll be great for the vast numbers of users who just need to add some basic CSS to customize their theme. However, for many of us, it’s an unwanted feature on our own sites for one reason or another. This tutorial is going to walk you through disabling this feature based on your situation.
Remove directly from customizer
If you just want to get rid of the custom CSS option altogether, the easiest route is to simply remove the control from the customizer. Dropping the following code in your theme’s functions.php or a plugin will handle that fine.
add_action( ‘customize_register’, ‘jt_customize_register’ );

function jt_customize_register( $wp_customize ) {

$wp_customize->remove_control( ‘custom_css’ );
}
You can also remove the entire section (which won’t show if there are no controls) using the following line of code. However, that may have the unwanted consequences of hiding controls that other plugins/themes have added.
$wp_customize->remove_section(
Source: https://managewp.org/articles/13985/how-to-remove-the-customizer-css-option

source https://williechiu40.wordpress.com/2016/12/08/how-to-remove-the-customizer-css-option/