Remove additional information (order notes field) at checkout

Paste the following code in the active theme’s functions.php:

/* Remove additional information (order notes field) at checkout */
add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );
function custom_override_checkout_fields( $fields ) {
     unset($fields['order']['order_comments']);

     return $fields;
}

Source: https://docs.woocommerce.com/document/tutorial-customising-checkout-fields-using-actions-and-filters/