Simplify WooCommerce checkout fields for free checkouts

## wc-free-checkout-fields.php
<?php // only copy if needed

/**
 * Removes coupon form, order notes, and several billing fields if the checkout doesn't require payment.
 *
 * Tutorial: http://skyver.ge/c
 */
function sv_free_checkout_fields() {

	// first, bail if the cart needs payment, we don't want to do anything
	if ( WC()->cart && WC()->cart->needs_payment() ) {
		return;
	}

	// now continue only if we're at checkout
	// is_checkout() was broken as of WC 3.2 in ajax context, double-check for is_ajax
	// I would check WOOCOMMERCE_CHECKOUT but testing shows it's not set reliably
	if ( function_exists( 'is_checkout' ) && ( is_checkout() || is_ajax() ) ) {

		// remove coupon forms since why would you want a coupon for a free cart??
		remove_action( 'woocommerce_before_checkout_form', 'woocommerce_checkout_coupon_form', 10 );

		// Remove the "Additional Info" order notes
		add_filter( 'woocommerce_enable_order_notes_field', '__return_false' );

		// Unset the fields we don't want in a free checkout
		function unset_unwanted_checkout_fields( $fields ) {

			// add or remove billing fields you do not want
			// fields: http://docs.woothemes.com/document/tutorial-customising-checkout-fields-using-actions-and-filters/#section-2
			$billing_keys = array(
				'billing_company',
				'billing_phone',
				'billing_address_1',
				'billing_address_2',
				'billing_city',
				'billing_postcode',
				'billing_country',
				'billing_state',
			);

			// unset each of those unwanted fields
			foreach( $billing_keys as $key ) {
				unset( $fields['billing'][ $key ] );
			}

			return $fields;
		}
		add_filter( 'woocommerce_checkout_fields', 'unset_unwanted_checkout_fields' );
	}
}
add_action( 'wp', 'sv_free_checkout_fields' );